well, I just wrote you a book on this, then got an "invalid session" message when I tried to post it, and hit back and it was gone. so here's the short version:
EQ: since in an instrumental guitar recording the guitar's the main focus, you want to keep it as uncolored as possible. Try flat above 120Hz, then slope from 0dB @120Hz down to -infinity dB @80Hz, cutting everything below, to clear up the low end rubmle while leaving the basic "color" of the gutiar tone unchained. do this for both rhythm and lead tracks, but differentiate the tones to seperate better in the mix by tweaking them at your amp while you record. Remember, keep them as natural as possible in the mixing stages, and try to get the "right" tones whiel recording at the amp. (I generally go more bass and treble for the rhythm, and less bass and treble but mroe mids for the lead)
Compression: The point of this effect, barring exaggerated special effects, is to remove the transient peaks from a signal that our ears can't even really process anyway, to help it sit better in the mix. So, I eyeball the typical average dB setting (estimate where the "valley" is between the peaks), then set a compression threshold slightly above that. Then, adjust the ratio to the point where you can begin to hear the signal compressing, then back it up a bit so you can't anymore. This generally brings you to about 4:1 or so.
Volume: A/B your mix against a cD with a guitar mix you really dig. There's no hard rule hear- for instance, the Smashing Pumpkins do a very guitar-heavy mix with lower vocals and drums, while Nickelback mixes the drums, vocal, and bass pretty high, while the gutiars are comparatively further back. It's ice cream, really.
Chorus: this robs gutiars of some of their "presence" in the mix. This is great with rhytnhm, and a light chorus can help a rhythm track sit back in the mix better. However, it's the exact opposite of what you want from a lead. Too much always seems to sound amateurish.
Delay: ideally, you want to keep the delay track out of the same stereo and frequency space as the lead. a multi-tap delay set to pan to one (or both, if you so choose) sides is idea, as is a low or high pass filter on the delay. This actually sonds more natural, anwyay" in the real world, certian frequencies are usually absorbed as the signal refrlects off a surface.
Pan: when in doubt, pan. I usually go 40-70% each side for rhythm tracks- i started doubling hard left and right, but a guy whose opinion on this stuff i trust infinately suggested going a little closer to the center, as it osunds more natural in the acoustic space, and saiung the outer ewdges for special effects stuff. Cool. but nothing, even snare and kick, should be absolutely dead center except the lead guitar (or vocal). Even slightly but inperceptably to one side is better than dead center.
Hope some of this helps...