You will have a list of things that you know that you like: nut width, neck profile, scale length, body size/weight, tuner arrangement (4 in line, 3 over 1, 2 + 2), pickup selection and general tone. Once these are sorted, it’ll be Martin and John’s expertise that’ll help you come to a final design and configuration. I’ve not had a Sei built, but I’ve had custom basses and guitars made before and the ‘mistakes’ I made were falling for aesthetics over ergonomics. Yes, you’ve got to want to like the look of it, to want to pick it up and play it, but it has to work as a physical tool first. Martin and John are serious people and they want a good product out there with a satisfied customer, if you come in with a similar attitude, it’ll be golden.