Best to find somewhere you can in roll it, get a good square cut ,then roll it up again. Roll it out where it is going with the excess up the wall and cut with the blade towards the wall. Will get some dug burns cutting that way but you will get rug burns eventually won't you?😏
Don't assume you room is square. Your walls straight. Roll it out, secure a factory side to the straightest wall and fallow the other wall, cutting the back of the carpet.
find a place you can unroll it, unroll it and fold it into a configuration that can be carried in and layed out in the room then rough cut it into place, get it secured then do the final cut in by hand .