Quoted from mrm_4:

Here’s what I did for my Bally Rocket and United Rio.
Flattened 24 of these things.
Drill a hole in a board
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Put the skirt on the board with the pin in the hole
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Lightly warm it up on low. Feel it with your hands periodically and push on it you’ll feel it soften up.
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When it starts getting flexible push it down to the flatness you want with something (glass, book, finger) until it cools.
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Flat as the board!
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This particular one the center is a little higher than I like. The more you press the center the more action the skirt has because the edge will sit higher off the playfield
I found when I was doing these is just a little heat will make these react. Don’t approach it like modern plastics that’s for sure. I blistered my first 2 skirts because I was too close and too long with the heat.