It is hard to deny that there is a lot of variety in 40k. From all the codexes to Forgeworld units, there is plenty to choose from. However, what if that isn't enough, and you want to make something of your own to play in games? Here's a few tips on doing so.
What do you want?
Think about this really hard. Do you want to create an entirely new unit with custom guns and armour? Do you want to represent a unit of character from a book? Do you want to make an alternative model for an existing unit (For example, think of the WWII German Tiger and the Tiger P). Once you've decided, it is a good idea to put it to paper. Do a few sketches, or write some fluff; something to give you a picture of what you want to make.
Count as or custom
Count-as is the simplest way of assuring you can use your custom unit. Have a look in your codex (or other codexes) and find a unit or model that pretty much covers your unit. For example, if you want to make a new Imperial guard Battle-tank, consider using the Leman Russ rule-set. Got a new stealth unit? Try counting-as scouts. If you cannot find a suitable set of rules, then you should think about custom rule-sets.
Model first, rules later
I prefer making a unit before making rules for it. This is because if you do the opposite, you may fall into the trap of making over-powered rules or designing the rules to be of benefit, which may alter your desired unit.
Make sure to gather all your needed materials, parts and donor vehicles, then get stuck in. If you need some help with that, check out this article.
Once you've built it, and decided not to count-as, now is the time to make the rules. Think about what the unit is designed to do. If, for example, it's a heavy battle-tank, think about giving it high armour, but making it slow. Picture how it would work on the battlefield and work the rules around that image. Feel free to use or invent special rules, but don't go overboard, because it may end up A) overpowered, and B) very complex to play. The latter may annoy you and your opponent. Remember, the more you give it, the more the unit should cost. If it helps, look at similar units and base your points cost on that. Also decide if its going to be a troops choice, a HQ, and so on.
Remember though, even if it's the most tested and balanced unit in existence, check with your opponent if you're allowed to use it. With this advice, I hope to see a ton of custom units flooding my inbox in the weeks to come.