Hi Nigel,
I use a ThinnerLine circle cutter.
Similar to that sold by others, such as DSPIAE.
I first cut a paper template to get the correct size.
Then cut the disc from the decal paper.
If the outer wheel covers have extended axle ends, I'll drill the the centre of the disc with an appropriate sized drill.
As most wheel covers have a conical shape, the decal needs to be slit across one side in a shallow wedge.
That allows the decal to conform to the shape and the cut faces to meet.
Then just apply the decal to the wheel cover.
Once set I cut into the opening for the wheel valve.
Then I apply a small amount of Tamiya X20A thinners into the hole and also around the axle ends.
This will melt the decal and conform it as required,
Mike
