I’ll just share some suggestion here, and some things I learn when making my own yoyo.
First of all, you had to have a really strong concept about what you want to make, in this case a yoyo design. This include the general shape and size of the yoyo, and the general feel you try to archive.
Next is to try any yoyo that close to what you want to make. I mean, lets face it, these day there are tons of yoyo out there already, and there’s a chance something is very close to what you want but not quite.
After you have a very strong concept of the yoyo you want to make, it is time to make a CAD model on the yoyo, I guess this is far better than going to a machine shop with a rough sketch, because; 1) the machinist can understand what you want better, and most importantly 2) you can visualize what the yoyo would become in real life.
And the last step is just find a machinist to make your design.
There’s many other things to do other than what I said here though.