Originally Posted by panda racer
Here's what I do:
If the wheel arches are marked:
I use rounded scissors for the first cut and then sand down the rough parts (if any) with 600 grit paper until the surface finishing is perfectly smooth.
If there are no markings for wheel arches:
I place the body shell on the car, using the front wheels as a reference, then I drill holes for the body posts, and I mark the wheel axle's position on the body.
After that I attach a X Acto knife to a compass and I use it to trace the wheel markings, using the wheel's axle position and the tyre's max diameter + 2mm to draw the circle.
Once all this is done I carry on with the first part.
Using the factory marked wheel wells doesn't guarantee the body will fit on the wheels properly, so I always first do a fit and eyeball where I would think the wheels would sit best, drill the bodypost holes and then follow on from there as explained above in the second paragraph. I use an Olfa compass. I then try fit the body and enlarge the wheel wells until I am satisfied the body won't catch in the wheels especially the front at full lock either way. I use for this operation a sanding drum on a drill press, so edges end up smooth as silk.
Tamiya bodyshells are very good quality in this respect and normally you can cut straight away as per their markings, but when using a non-Tamiya chassis it is worth a check especially for the body posts.