It’s hard until you do it a couple times. Airports have certain regulations depending what aircraft go where of course and there’s some loose literature surrounding certain airports and FAA/global requirements and regulations.
Seattle actually has documents online for their ground markings and what lines go where and how big/long each line is which is fun. For example on PHL here, I haven’t found anything online like Seattle’s but if you look at nearly every other US Airways owned gate from 2005-2015 you’ll see they’re all the same basic shape. Anything up to an A320/21 on this diorama is the “triangle with a box top” as I’ll call it. I’ve made the max width on those to be ~1 7/8in on each side of the centerline, as in scale that’s generally about how wide that space is to accommodate those aircraft. The 757 gates change by adding the flat perpendicular lines in front of the engines, and also increases overall width to ~2in on each side of the centerline. Some airports are even a touch bigger to specifically accommodate winglet 75s easier. The 767 gates are the 757 shape, but bigger now! Every US Airways gate (mostly) has always looked like these which becomes easy to replicate as you do things. Certain gates will also have their own quirks depending on space requirements and all that fun stuff which does make it more difficult.
I am home now and want to work on this anyways so let me do a new gate and try to remember to take photos during the process!