Foiling the wheelwell moldings has often proven difficult as is thin and on a curve. Many times there is not a molded in lip that is detectable under the paint. I used to just freehand it and hope for the best.
I now use the technique described below which makes it easier to get more consistent results. The same technique can also be applied to other areas such as the gutter molding.

The first picture shows the subject – a wheelwell of a 1965 Galaxie. As you can see there is no molded in lip that is visible.

Tamiya offers a flexible masking tape that I will be using. The tape is stretchable to conform to the curves.

Cut a strip of tape long enough to place along the edge of the wheelwell, usually about 6 inches. The tape is put along the edge leaving a small line of paint showing – this will be your molding. Stretch the tape as needed to conform to the curve.

Once applied, take a good look to be sure the gap is thin enough and even. The tape can be adjusted easily at this point.

Apply and burnish the foil as you normally would.

Note that the edge of the tape is easily visible under the foil. A sidenote – I use a magnifying visor in order to help my aging eyes clearly see the edge.

Cut the foil along the edge of the tape. With a light touch, you will find that the tape will help guide the blade.

Here is the finished molding. Later I was able to trim off that slight bump at the top of the wheelwell.

Happy foiling.