This will be about the shortest guide ever, and it's nothing that I made up. It's an old trick for drilling a specific distance into something without a drill press. First, determine the length you want. You can use a ruler here, but in most cases it won't be necessary. Then, put electrical tape (any tape really) starting where you want the drill to stop drilling.
Then, making sure your drill is square, drill into the item in question until you reach the tape. When the tape hits your workpiece, stop drilling. There! Your distance is secure. I just thought that I would share this with anyone who didn't know it already.