it isn't particularly hard to do. get a dremel with a carbide cut-off wheel or multi purpose bit (anything that will cut through skeleton cleanly); a fine hacksaw will do too i think. if you can just cut off a branch that would be best, but if yours grows like the typical mound like most read on.
1)make the colony close up completely
2) remove from tank (it will slime a lot) and think about where you're going to cut
3)have at it and try to get as clean a cut as possible. this is obviously the most important part
4) super glue the skeleton to a plug or something. finito
try to keep each frag to at least a full polyp; i suspect half polyps might actually be fine though.
Acros aren't warriors and duncans can be drama queens, but they are very hardy corals. no guarantee, but if it's just closed up, it might actually recover without fragging. I would give it a day or to to see if it gets worse; if it does, then chop out the bad bits. is the tissue actually receded and dead?
here's some fragging pics (not mine). scroll down about three quarters. title: "Duncan frag time...":
http://blogs.frags.org/member.php?uid=4043