To do this exercise, you'll need a rack or a chin-up bar that allows you to set another close underneath it. Alternatively, you can also use the metal ladder of playground apparatus (that works quite well, too).
Set the racking pins (where you would normally rack the bar on a squat, for instance) to the highest point you can on the rack.
Now move the safety rails up to a point that is about 8 to 12 inches below that (you can play with this height when you try the exercise to get the best dimensions for you).
Set a bar on the top racking pins and set another bar on the safety rails, pushed up against the uprights. It's going to look like two rungs of a ladder.
In my setup, I load the top bar with weight plates to keep it from moving.
If you have a power rack that has pins you can slide in and out of the frame (I don't - mine are intergrated right into the rack) you can also more easily get this set up by setting one safety rail in the top set of holes then the other safety rail in a set of holes below (making your own ladder, essentially). That's the easiest way to set this up and how I used to do it when I had access to a rack that I could do it with.
Now get in the rack and take a close, underhand grip on the top bar. Your forearms should be braced up against the bottom bar, just above your elbows. Your arms should be straight at the bottom.