You start with figuring out the blank, to help pick the blank know these answers:
What are you fishing for? Lure weight?
Casting or Spinning?
Length?
Reel is nice to know as well, especially for heavy saltwater.
From there the Rod Builder can make recommendations if you don't already know which brand you'd like.
A good builder will start there, then they’ll ask questions like:
Budget?
What type of grip do you like? (Full or split, cork, eva, carbon fiber or something else)
Also good to know palm to the meat of the forearm (where you like the butt to hit). If you have a rod that you currently like how it fits you grip wise then you can measure from the reel seat to the butt.
Color Scheme? (for the thread work)
How detailed or minimal do you want the thread work? Basic or Fancy.
From there they’ll make recommendations on the reel seat and guides. If you already know what you like great but if you don’t, don’t worry that’s their job to help guide you along the journey of getting a custom rod built.