It doesn't really just "slide" into the channel. There are slightly contoured edges that are a little thinner than the base material and you pinch it into a "U" shape and it pops into the upper lower channels to lock it in place. When done, it is tension curved on the outside, that's what keeps it in place. If it wants to lay flat it's to narrow and you will need a wider material.
If you put it on with the wrong aside out ,it can pop out later. Cut off a small sample and see which way works best. In looking at a flat piece the edges will point slightly UP when the outside is UP.
Start at a door and work you way around the trailer. If you get a 50' roll you can do it in one piece.
Here is a pic of what it looks like flat (R), with the outside surface UP, and how you pinch it into a "U" shape (L) for inserting into the channel. You can see how the edges fit into the slots.