There are several elements to consider when designing packaging for your product.
Like any good design, packaging tells a story. It’s also a sensual experience, literally engaging us through sight, touch and sound (and possibly smell and taste, depending on the product/package). All of these details help us understand what the enclosed product is for, how it should be used, who should use it and, maybe most importantly, if we should buy a product or not. 
Design / style
What is the purpose of the box? Your design will depend on if it's a once off fun promotional gift box to promote a new product to your VIPs, or if its a retail packaging box destined for the retail or supermarket shelves. These are practical design considerations.
Also think about who is the target market and what design will attract your target market? also think of fonts, colours, shape.
What is your brand image? eco / funky / innovative?
Try to create a packaging design that is functional, attractive and unique to help you stand out.
Packaging is the last message a consumer sees and a last chance to convince them to buy the product. Clarity, honesty, authenticity play an important rule in this process. 
What size of box do you need?
If you are packaging a product/s then lay the product or products out on a table just as you would like to see them displayed in the box. Measure around the outside of the products - that way you can find out the size of box you need. Also tell the box manufacturer the size of each product and send them a picture of the layout - just so they can double check your measurements are correct or that the layout is the most attractive and / or effective way to present the products in the box.
Paper stock for the box
Is your product delicate? If so, you would require sturdy stock such as a rigid board box and/or inserts to hold the product in place. These insert options are discussed below.
If your product is a non-breakable product - just as face cream in a plastic tube, a simple flat pack box with flap lid in thinner artpaper stock would be suitable. However if you have a high end product, a thick rigid board box would create an element of luxury for your product.
Discuss with the box manufacturer what you hope to achieve with the look of the box, your brand image and the product that is going inside. They will be able to provide you with some options based on what would work best for your unique product and project needs.
Inserts can be used as a presentation tool or as a protective measure - but often both are achievable. For example a wine bottle gift box can have a protective foam insert and be covered with satin inlay (see the example of the black box with satin below) - meaning the glass bottle is protected, but it looks luxurious, surrounded by ruffled satin.
Inserts can be used to hold products in place in a larger gift box to present them beautifully. For example in the blue gift box below the products are protruding out to the custom coloured card insert.
If you require a box with inserts, think about the practical function and aesthetics. Discuss your needs with your packaging supplier. If you require inserts, that often means the outer box size will need to be larger. So be sure to advise the packaging manufacturer of the size of the individual products that you are putting in the box so they can ensure the outer box is large enough to hold the products and provides some space for the foam or card insert border.
For complicated insert design shapes you would likely require a Packaging Designer to create the dieline template. But check first with your box supplier if they can supply it or if it is beyond their capabilities.
Text, logo, images, legal information and a barcode considerations are important to discuss with your Graphic Designer.
Think about special finishes like spot uv (gloss treatment), debossing, embossing, foiling (see gallery below for some examples).
The first thing you need to consider when discussing with your Graphic Desginer who is creating the artwork, is making the purpose and brand of the product crystal clear. You can do this by prioritising the logo, product name and product description on the front-facing side of the package. This is without a doubt the most important thing you can do in your design—if what the packaging actually contains isn’t clear, the consumer will skim over it and buy something that’s more obvious.
You also need to make sure you are designing for the target consumer. Many designers get swept up in designing packaging that they would likely buy, which no doubt makes for a pretty result, but this could end up alienating your target market. Do some market research into packaging in the same sector to get a sense of the colors, graphics and typefaces that feel appropriate for the product’s market.
Another great tip is to design your packaging from an emotional perspective. We interact with dozens of packaged products every day; because we are saturated with visual information, it takes something with a real emotional pull for us to pay attention to it and ultimately purchase it. Emotional design can take many forms, targeting feelings of aspiration, nostalgia, joy, or even physical attraction. Keep the emotion at the forefront of your design and you’ll find that consumers are more naturally drawn to the product. 
Your packaging supplier can often supply the dieline template of the box so your Graphic Designer can just pop the artwork on the dieline. Otherwise, you would need to find a Graphic Designer who also has packaging design skills to create the dieline for you.
What is your competition doing? if your product packaging is going to be sitting on the shelf in among your competitors you could easily be overlooked.
What can you do to stand out whilst still keeping to your brand image and attracting your target market.
Shelf impact is something you need to test and explore in your designs. You can do this by imitating the placement of your design on an actual shelf and surround it by other products (for best results, use several rows and columns of each product). The more distinctive it looks, the better it sells.
Note: you will be amazed at the results – sometimes the best looking design will simply blend in and become invisible, while more simple designs “pop” in this environment. 
Feedback from your target market is vital to ensure your packaging communicates the right brand image and information, is attractive to your target market, holds the product well and attractively.
You might not be your brand's target market - so don't assume you know what your market likes. For example if your target market is teenagers, be teenagers these days might be attracted to different colours, designs, images than you did when you were a teenager.
The same with targeting different nationalities - different colours represnt different things. You dont want to be trying to break into a market and have your packaging represent death and misfortune. Many people would be put off.
Market research can be employed to ensure that the packaging is on track with its intended objective.
Early stages of package design:
Before making some decisions on the direction of the package design, qualitative research such as focus groups can be used to explore what consumers within the target market are looking for. Opinions on currently available competitive designs can also be collected to estimate what package features are desired, which ones need to be improved, and what the market’s overall expectations are.
Either in conjunction or as an alternative for qualitative research, a choice-based conjoint can be applied to measure the level of preference of a design. In a conjoint exercise, simulated packages can be created for research purposes based on a pre-defined set of dimensions (e.g. colour, package shape, text font, logo, etc.) and within each dimension a pre-defined set of levels (e.g. blue, green, red among the colours; square box, cone, cylinder among the shapes; etc.). These virtual packages are then tested in such a way that respondents’ preference for each level is determined, as well as determining the impact of each dimension on their overall packaging preference. This research can direct designers to a small number of options that would be most compelling to the target market.
Later stages of package design:
Once a shortlist of concepts has been established, these can be tested among consumers. This can again be done in either a qualitative or a quantitative framework, or both. Qualitative research can only be used if the concept packages can be viewed, manipulated, and handled in-person. In cases where this is not feasible, an online quantitative survey can be employed in which the concepts are presented. The testing of the concepts would first be in a monadic fashion, i.e. the sample is split into equal sub-groups, with each sub-group presented one concept on its own. The concept is then rated it on a scale on various measures (appeal, uniqueness, etc.). Then, other concepts are presented, and respondents are asked to compare which concept most stands on those same measures. Each concept can also be tested in a word association exercise (“old”, “novel”, “daring”, “boring”, etc.), and then mapped against each other on these descriptors. 
 Kelly Morr https://99designs.com.au/blog/tips/ultimate-guide-to-product-packaging-design/
 Grace Fussell https://design.tutsplus.com/articles/the-beginners-guide-to-packaging-design--cms-27380
 Peter Vukovic https://99designs.com.au/blog/tips/6-rules-of-great-packaging-design/
Clara Cassidy, Founder and Marketing Manager of Custom Printed Bags & Boxes, is a marketing professional with years of experience in branding, promotions and events.