Forming a Perfect Square From a Large Roll of Paper

[AquaMan]

This is probably the easiest thing to do as I haven't been able to find a post regarding this.
It's probably very obvious, but my usual method of folding down a triangle and cutting off the bottom to make a square only works if I know for sure that I'm working with a perfect rectangle to begin with.

Can somebody share their method for cutting a large square from a large roll of paper?

[Gerardo]

I bought the biggest set square I could find, so I could trace right angles on each corner. It worked very well; I was able to cut a 150 x 150 cm (5 x 5 ft) square.

[sheeptortoiseorigami]

Grab a corner of that paper roll and pull it across to the opposite edge, forming a triangle, then snip off the excess, following the edge of the triangle. This is what I ussually does