That is the worse part for me, for some reason. I seem to have some mental block about putting borders on. I have two quilts ready to put the borders on and it's hard to make myself do it.
I have a 4' long metal straight edge, a builders/carpenters tool, and a very large metal square that I use. I just have a thing with worrying about cutting long strips straight.
The last border I cut I folded the 3 yards in half so it was 1 1/2 yard long, laid it on the floor, and marked the line with the two metal tools. I then slid my 24" mat under the fabric and rotary cut it using the 4' straight edge. When I got to the end of the mat I very carefully slid it further up the line to cut.
How do you do yours?