[Synchro909]

I've trimmed several cars and covering the "panels" before fitting them is not the way to go. The way they were done in the day by tradesmen was to tack the edge of the fabric face down to the pillar close to the edge, then the fibre board panel is placed in position and tacked down. Once that is done, the fabric is folded over the panel so that the front of the fabric is now facing you and the edge is tucked under the board with a thin piece of steel like a rule or spatula. This obviously covers the heads of the tacks used and leaves the whole thing SECURELY attached to the pillar with no tacks or nails showing.
Difficult to describe with only words but I could find no video demonstation on the net.