guitar with amp or acoustic? Will there be other things recording through mics at the same time...vocals or drums? A guitar amp needs isolation, or it will bleed into other tracks, if you are singing or have live drums. Or, if you have live drums and this guitar is acoustic, the drums will probably bleed into the guitar mics.
A guitar amp by itself might not need to be isolated, but unless you want the room to be there for ambiance, you should make a kind of baffle area, mostly sides and maybe top, if you need it for isolation. Thick blankets would work, draped over a rope or board, suspended from some kind of support. You would want more on the wall, behind the amp. You could also build an enclosure just for the amp, and give everybody headphones. Pad the inside of the enclosure and make sure not to leave the amp in there for too long, if it gets hot. A nice box with padding on the inside that fits right over the amp would be fine, well, if the amp is small enough. A large speaker cabinet might be best positioned in another room, still with blankets around it, to keep the sound isolated.
Mic it with a good mic, maybe Sure sm57 and let the guy play while adjusting where the mic sounds best. Start with it right at the edge of the speaker cone, a few inches away. Then, adjust to taste from there. Of course, any enclosure or blanket you add can't buzz or vibrate, so think of that.
If this is an acoustic guitar, it still would need an enclosure, but the player would be in with the guitar. The same blanket arrangement would work, but the louder other things (like drums) could still be too loud and you might have to record them separately. If you just record the acoustic guitar alone, you could put blankets behind and to the sides and leave the front open. Then, you could use two microphones, placing one a distance away, to get room ambiance. It all depends on the sound you are looking for. Try different placements, and different baffles to get the sound you want.
The basic idea is to experiment with mic choice and placement, until you are familiar with your room acoustics. I have blankets on all of my walls that I can roll up or down, depending on if I want room ambience or not. You can buy baffle boards or make your own. Thick blankets work just as well. You can drape them over the backs of chairs, if your player is sitting down. Then, he can look over the blankets.