This is way easier with the bezel off the watch.
Lay the bezel flat on a bench. Get a piece of thick plastic or smooth wood about 1/2" wide and 1/8" thick and 6" long or so. Wrap it a couple times around with a soft cloth.
Dip a finger in some liquid soap and run your finger along the insert edge. Place the insert into the bezel and snap it into place over the 85% of it that will go. Hold pressure on the insert at the top and bottom so it can't pop up when you do the next step.
Use your wrapped tool to force the remaining 15% down into the bezel well. Exert pressure first in an inward manner to overcome the lip of the well and then kind of roll the tool down on top of the insert. It can take a significant amount of pressure to pop it in.
Pro tip: Before you press it fully, and with the bezel on the watch, identify where the triangle needs to go so that it ends up centered on 60 with a click detent. For example you can put a Sharpie dot on the triangle at the exact middle of a bezel scallop. It sucks to go through all this and have the insert half a click off. Alcohol takes the mark right off. Or note where the top two triangle tips need to go relative to bezel teeth or scallops.
When you go to reinstall the bezel, put the bezel tension spring (big washer) inside the bezel then thread the click inside it as you slide the bezel into position. Rotate it gently to make sure nothing is caught wrong before pressing it home.
Good luck
Just did this myself - have all 3 inserts.
Note that the apparent position of the triangle shifts with viewing angle. Didn't want you to think it wasn't centered, haha.
