Once the resin is covering the whole tray you’ll notice air bubbles. The best way to get them out is to use a heat gun or a hair dryer. It’s actually hypnotic to watch, they just disappear! I got the heat gun at Michael’s with a half off coupon for under $15. (It’s with the gift wrapping stuff, to be used with shrink plastic.)
Note: I wouldn’t usually say go full screen, but in this case the small size doesn’t show off the bubble popping action.