I buy mine pre-cut. They are always exact that way and everything goes together very easily. I did try cutting my own using old file folders and my rottary cutter but it is really hard to get them exact. The pre-cuts are quite inexpensive and many quilt stores carry them. I've also ordered them online without any problems.
The fabric I do cut with the rotary cutter. I first cut strips the width required then stack two strips and cut out the shapes using the 60 degree line on the ruler. Some people just cut squares and fold the extra to the back of the paper.