I fold the fabric twice (make sure your rotary cutter is big enough to handle more than three layers of fabric).
The first fold is the way it is put on the board, and it may or may not be straight. If the fabric was loaded with a bunch of wrinkles, you have a choice of ironing it out, or throwing the whole fabric into the dryer with a wet towel and tumble for twenty minutes or so. Take out immediately and start the folding process. As mentioned before, get the selvages as close together as possible, along the length, but rest assured, your chances of having them line up exactly are pretty slim ... that is the first fold - - go to the folded side and begin smoothing the fabric so that it is laying straight and flat (at which point, you may or may not have selvedges matching) - - not to worry, your goal is to have your fabric straight and flat. I then fold the fabric a second time. If it is a great length of fabric, I go get help, but the point is to not allow the first fold to get away from its determinded correct position. If you practice on a fat quarter first, in time, you will move up the size frame until you are adept at any length you can get help with or handle on your own.
Some might ask: But, aren't you afraid that you are going to get across the grain? Well ... no ... since many old quilts that have lasted more than a hundred years have been found to have squares made on almost the bias ... just treat your fabric with respect, encouraging it to lay flat and smooth, don't pull on it while it is trying to sew through the machine, and you won't notice the off grain at all ... two inches is two inches, no matter what direction it get there.
There is no grain god waiting to strike you down because you got cross of it <g>.