Quick 1-2-3 pic tutorial of how I start most of my custom front lips.
Always need a base, then just add on from there.
Take whatever car you want the lip for, place bottom of front bumper on styrene sheet (I used .030 here).
I used a Microlon .005 pen, but a mechanical pencil is small enough to trace accurately enough too.

Just trace the stock body/bumper line and where the bumper ends at the fender well.
The lip size itself, I just eyeballed it and freehand drew the line, using the first line I traced as a perfect reference.
You can see with the styrene exposed, I have marked where I’ll glue and also the lines in the front and behind the bumper, all the ensure I stay within my eyeballed specs.

front lip

Tutorial Courtesy of Julie Jnx ScaleTunedCustomz