I'd also recommend picking a maker that already makes designs similar to what you want.
Don't ask a maker who hand forged carbon steel , and heat treats in a coal forge to make a stock removal knife out if wizz bang super steel hest treated to 65 rc..... (unless they are set up for, and know how to do it).
Also, super fantasy designs, be prepared to pay up front. By that I mean, that if your design is so out there that no one else is going to buy it, the maker really may have to take money up front to protect their time and work output.
I've seen makers try to sell outlandish designs where the cmoriginal buyer backed out. The maker usually takes a huge loss at that point.
I've had a full custom, where the maker and I worked our design, size, material, number of blades, steel, strength of pull, etc, etc. No money up front. Super Fast turn arround time, and nothing but absolute satisfaction on the best knife I've ever held. (But, I knew the maker before he started making customs).
What ever you do, they to research feedback on the maker, and get info/pics, and if at all possible handle some of his work if you are able.
I have customs that I would have never bought in the secondary market if I had been able to handle them in person.
Avoid big deposits. 10, 15, 50 dollars to hold a spot in line (and discourage looky-loos who are not going to pay is one thing). But even a small deposit where the maker is 8 years behind on orders is going to gall you if it does not work out