Quoted from SteelTigers:

Just installed the Gen 2 Guardians Cliffy set. Started with the trough ball eject and shooter lane. The shooter lane is rather easy, you just unscrew the wood rail from the playfield and gently pull up to make a gap, slide in the shooter lane protector, line up the holes for the screws then tighten them back up.
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The center drop target requires you to remove the drop target assembly from the bottom of the playfield. Since there was a already a hole to fasten the protector, I opted to drill a hole into the bottom of the playfield to secure the drop target protector.
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If you use a depth gauge on your drill bit, you'll never punch through the top of the playfield. I used a piece of copper tape to mark my depth.
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Screwed in the protector, re-mounted the drop target assembly, done deal.
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Last but not least, the trough. All you do here is loosen the 6 wood screws that secure the trough just enough to make a gap between the playfield and the trough, slide in the protector between them and re-torque the screws.
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Now it's time to fire up the game for the first time...