Raise the arms so that they are straight across (halfway), and slide the rollers into the channel from one side. You will need to have the regulator unbolted from the tailgate to do this, as you will have to move the regulator side to side.
There are two styles of window channels, one has a solid channel all the way across the bottom of the window, the other has two separate channels.
If you have two separate channels, align the rollers on one side of each channel and slide it in.
If you have a solid channel, insert one roller on one side, then slide the entire regulator assembly to the other side so you can slide the other roller in.
...I hope that makes sense... I've done it on both types, so I know it's possible. The trick is to move the regulator, not the window.