step 1: have wiring diagrams in random loose format: pictures, scans, PDF, text.

step 2: have ECU connector pinout in google spreadsheet form, with explicit function of each pin, often with analysis of multiple vehicles with similar pinout. Good example https://docs.google.com/spreadsheets/d/1H0cZPAJFbpprgSu1Y8BiAYzXbqddvIn-Hhod4QCVQwk

step 3: breakout board for that connector

step 4: pinout data entry into KiCAD

step 5: proteus or else adapter for specific vehicle, based on steps 2, 3 and 4

step 6: full plug in board like Hellen