Sorry! I missed this question last week...
A "sled" is a piece of board with mounting adhesive applied to one side that has the release paper left on - this make a surface that the laminate won't stick to. You put the piece of vinyl down on the sled, then take a piece of laminate cut a bit larger than your printed piece and start it was printed adhesive vinyl you were going to mount.
Here's a real quick example:
View attachment 150381
Also,
found this video...it's a small manual laminator, but it's all the same idea. Looks like someone else posted the same video! I'll leave it here anyway. Hope this helps!