The ski probably just had bindings put on with #10 stainless sheet metal screws.
If you need inserts, then you do probably want to epoxy them in, the epoxy is on the outside of the insert. The appropriate hole size is specific on the insert.
The best process is to put masking tape on the top deck of the ski so it drills clean, and avoids epoxy mess. You want to drill your hole only as deep as needed, and nice and square to the ski's top deck.
Then take a bolt that fits the insert (8-32 probably) put two nuts on the threads of the screw, a little vasoline, and thread the insert on, then once the bolt is about to protrude through the insert, run the nuts down and torque them against the insert. Mix up your epoxy and put a bit in the hole, then thread the insert in using the bolt as your tool.
Let the epoxy harden, then back the nuts off using a small wrench while holding the bolt still, then remove the bolt.
Epoxy mostly functions to seal and prevent the inserts backing out of the ski.
Once they are all done peel the masking tape off.
But you can usually just use screws and avoid the inserts.