The first stage of product design is the concept stage. This is where you first start to think about the product and form ideas about what it will do and what it will look like.

You could have a 'brainstorming' session where you generate ideas for the form and functions of your product. You can make quick sketches, lists of functions, lists of parts etc. Don't rule too much out at this stage and don't go into too much detail.

You should check to see whether there are any relevant standards or guidelines for your product. These may have a large bearing on your design, especially where safety features are concerned. (See the topic on product evaluation for more details.)

You have a product in mind so you now need to know who is likely to use it. You may know exactly who it is aimed at, for example, you may be designing a toy for young children, or there may be a whole range of users with different abilities and needs.

A 'user group' is a group of people who will use your product who have similar characteristics or who may use it in a similar way. You might have more than one user group for your product.
 

You should write a 'task description'. As its name suggests, this is a description of all the tasks that someone can carry out with your product. This can be very short for something like a saucepan, or quite long for a mobile phone, for example. Whatever you are designing, you should try to make the description as simple but as complete as possible, as this will be part of the definition of your product. Try to carry out a 'task analysis' too, which again, as its name suggests is an analysis of all the tasks that someone will carry out. Make sure that you look into and write down all the steps that someone has to take to use your product. Again, this will help you to define your product and might show up any potential problems at a very early stage.

You may have a use for your product in mind, but you also need to think about all the ways that your product might be used carelessly or abnormally. You should make sure that your product will not become unsafe in these cases. It will be difficult to insure against everything that people might do with your product but you should make a guess and adjust your design accordingly.

There might be times when there are other factors to take into consideration. For example, will your product be used outside, in which case it might be wet or cold or dark? Can it still be used safely in these conditions? Does it need to be waterproof? If your product has buttons, do they need to be larger or have more contrasting text to make them easier to use in the dark? 
 

Ask other people in your 'user groups' what they think of these products. You might want to use a checklist if you have lots of users that you can ask. That way you can see how many of them agreed or disagreed on whether they liked certain features or found them easy to use (for more information, see the checklists topic). Ask them how they might improve the products. If the product is something that they can use, watch them using it. Do they have trouble or make any mistakes? 
 
 

You now need to match your product with your users and come up with some designs. You might need to take into account your users' body dimensions and their strength - this is called anthropometry (see the topic on anthropometry for a full explanation). For example, small children or people with reduced dexterity such as elderly people with arthritis, will need larger, chunkier controls, buttons or handles than quick fingered teenagers!

Don't forget to think about all your users, including any that might need to move or maintain your product. If you are designing a piece of furniture, for example, you might need to think about how someone will lift or move it to clean around it.
 

Make your mock-up full size if you can, out of wood, card, foam or any other cheap material but make it as realistic as you can at this stage.

Get people to use your mock-up in as near normal a way as possible. Tell them only what they would reasonably be expected to know beforehand, then watch them using it. Are people using awkward movements or postures to use your product? Are they having to bend down to see or reach some part of it? Are buttons, catches or handles fiddly or difficult to use? (For details on handle design, see the tools topic.) If people are making mistakes, can you design them out? For example, floppy disks were designed so that they only fitted in disk drives one way - in the correct way.

Are people getting appropriate feedback from your product? For example, if they press a button, do they hear a beep or feel a click? If there is no feedback for this sort of action and there is no other visible sign that something is happening, they may get frustrated or keep performing the same action over and over again. For example, if your computer crashes, you may keep pressing the same button repeatedly, even though you know that it will have no effect if it hasn't worked the first time. 

If you haven't thought about instructions, now might be a good time to check out what instructions people might need. Is it obvious how to use your product? Can people find all the functions and use them properly? If you need to provide instructions, they need to be appropriate to your intended users too in terms of how they are presented and the language they use. If you find that you need to provide lots of instructions, maybe you should think about simplifying your product...