To get the parallel cut, most times marking with a pencil (or even using a panel gauge) is sufficient, especially if you clamp the rail to the board being cut (assuming it's wide enough for easy clamping). The clamps can help prevent the guide rail from drifting, but it's usually not a problem. You just have to be a little careful, that's all, so take your time to get everything aligned. Use a pencil with a chisel shaped tip or a fine mechanical pencil, as a fat dull regular pencil can be too thick (and the line can even vary in thickness as you use it). A really fine ballpoint pen also works.
You could also use the razor blade/glazier's point method described elsewhere here on FOG. Basically measure the width you want, then tap razor blades in at the right distance (single edged blades, obviously). Then, slide the guide rail over until it just touches the razor blades. Remove the blades before sawing, obviously.
If the board being cut is narrower than the guide rail, but still not super thin, you can rip it by placing another board next to it with the same thickness, to support the back of the guide rail. You just need to make sure the board being ripped doesn't move when you're sawing (tape it or clamp it in some creative way that's safe).