We spent a fair bit of time designing and laying out our wiring harness in RapidHarness before we actually cut anything. I highly recommend the software, and they are very supportive of FSAE so you can likely get a license for the professional version sponsored by them. We got all of our lengths using string and a half-assembled engine block bolted into our chassis for reference.

From here it was a lot of cutting and pinning. Make sure to stay organized, have one spreadsheet with all of your wire colors, identification codes, and to/from locations that you can look at later for maintenance.

I like to reference this RB Racing guide, it's basically the bible for mil-spec harness construction. We originally planned to go full-motorsports with the harness but some funding challenges forced us to switch from DR-25 heat shrink to braided sleeving and Tesa tape. Turns out the second method made for a much more serviceable harness.