Hey dtb.....Guernica can give you spedifics 'cause he uses a 788 but the functions you want to look at are the Cut and Paste (or move) function and the Wave-form editing. They look scarey at first but there's nothing to them. Take a track of guitar and put it on two tracks of your 788 and play around with these functions and you'll get it in no time.
So now you understand the functions.....how do you use that to sync up someone elses parts that they send you?
Lets say you have a stereo mix you want me to add sax to. You put a beep a couple of seconds before the music starts and send me the rhythm track with the beep on it. I simply record it into my recorder on tracks 1 and 2 and play along with it recording my sax on track 3. There's
no syncing required of me....I just play along with the rhythm track you've sent me. The only person that has to fool with syncing at all is the originator of the tune who has to assemble the parts once they are finished.
So I've got my sax part done........I then take that and send it back to you with the rhythm track hard-panned to the left and the sax part hard-panned to the right. Whether we use Mp3s, or wave-files or mail cd's, it doesn't matter....the process is the same.
Now you get the tracks and you record them to your 788 on any two available tracks. You try to get them in time with the original tracks as much as possible but it's probably not perfect. So the way you fix that is to go to wave edit and look for the beep. You'll be able to "see" where the beep is timewise on your screen. Since you've presumably already looked to see where the beep was on the original tracks , you simply "move" or "cut and paste" the two tracks I've sent you to that spot. All you're looking at is the beep but the rest of the tracks are moved right along with it.
Lets say that on the original tracks....you've determined that the beep falls on 2/10/002 (just making up imaginary numbers) and once you've recorded the "incoming" tracks that beep falls at 2/25/004. You take and "move" the entire two tracks to where the beep falls on 2/10/002 just like the original beep. Now everything on the incoming tracks is in perfect sync with your original tracks because you've made it start at the exact same spot by using the beep to line them up. Once you've done this....you no longer need the "incoming" rhythm track so you just delete it. Now you're left with just my sax part on a seperate track in your 788 just the same as if I came over to your house and recorded it. With the added advantage that you didn't have to clean up the house.

It looks far more complicated than it is......One thing to remember is that for anyone who's simply playing stuff for someone else, syncing doesn't come into the picture. If you were gonna play some guitar on one of my tunes.....you wouldn't have to do any syncing.....just play along with my tracks and send it back to me panned.
Whew! I tried to write this post last night when I got in from the gig but I had a buzz and kept getting lost. So I gave up 'till today.

I hope this helps. Like I said, Guernica could give you specific commands to do this 'cause he has a 788. I use a Fostex HD-1624 so some of the terminology might be a bit different but the basic process is the same.