It's a good idea to have a very solid sense of what you want in the general design of your vtuber to be before commissioning the full model! Vtuber models can get pretty expensive, so you'll want to be able to express your vision as best you can before comissioning one!A good way to start the process is through simple character creators, like on Picrew or even in certain video games! You can then gather references of other characters that fit the general vibe of what you're going for. If you're decent at art yourself, you can even draft your own design as well!Once you have a general idea of what you want your character to look like, I'd reccomend comissioning at least one good picture of your character before taking it to a proper vtube artist. Some artists will even do reference sheets for a character, which can help immensely with keeping your character consistent!Once you're through all this, you should have a pretty rock solid idea of what you want your character to look like, and you should have the materials to communicate that vision to a vtube artist clearly! With all that info, the vtube artist should have no problem making exactly what you want :D