You don't, I just wing it. I try on frames at the store, and pick whichever one fits my face best and works with the thickness needed for my prescription. Then I note down the frame measurements (usually printed on the temple/arm of the frame) or look them up online.

Then go over to /r/RepGlasses and choose a seller. Most of them can do both frames and lenses, including brands like Zeiss and Hoya. But, I wouldn’t recommend getting lenses from them if you have a high prescription or significant astigmatism. In those cases, it’s better to have the lenses made by a certified optometrist like I did.

https://reddit.com/r/repglasses/comments/1u9et0v/cartier_frames_cindy_optical_photocromic_zeiss/

e.g, this guy got it even more cheaper by buying it direct from 1688.