rrands, yes - cut it to shape with just a little over all around (ideally, it will be up on the wall a little all around the pool), this will keep 'max' heat in the water.
I feel obliged to remind you to remove (at least part of the cover) for an hour or so every other day to let 'things' 'gas off' so you don't run into some problems that can arise by keeping the pool constantly covered for days on end. [I don't fully understand chemestry, but apparently letting the pool 'breath' regularly helps keep things on an even keel]. Chemgeek could tell you all the interactions that are involved, or you could 'search' a few of his posts on the topic and get an idea of what I'm trying to say.
If you need any more help, feel free to ask (obviously - the site's free )