Basically, screw the die in your press/turret like any other die. The difference is, you back off the decapping pin a bit, lube a piece of brass, stick it in the press and run it up into the die. At this point, turn the decapping pin adjuster down until you feel it touch the brass and tighten lock nut. Size ALL your brass at this point. Once sized, trim all brass .020 less than SAMMI. Chamfer/bebur the brass. Now, clean one piece of brass if needed, relube, stick it back in the press and run it up into the die. loosen the lock nut and adjust the decapping pin until it touches the brass and then tighten lock nut. Normally, you have about four threads above the lock nut at this point. If you buy some new brass, this process will need to be repeated to get the new stuff set like the brass you already have.
Here's a link to the instructions:http://www.rcbs.com/downloads/instructions/XDieProductInstructions.pdf
I've been using these sizers since I started loading and love them. For instance, with my 270WSM, I trimmed my Win nickle brass to 2.09" and after several firings I checked the brass length and the ones I checked had grown around .004 to .005 and that's it.
Now if your OCD and just can't tolerate your brass not being all equal in length, then the extra cost of this sizer is not for you.