I've been playing off and on for about a year (and a half), and I'll go ahead and tell you how I approach it currently, how I approached it initially and what I think is the best way to go about learning this cool instrument all on your lonesome.
How I used to practice and why it didn't work:
So, I have a lot of hobbies, and the list is ever growing. More often than not, theses hobbies revolve around intellectual skill, usually mixed with some form of artistic expression. Guitar only really fits half of this bill. It is an artistic thingy, but it's a motor skill rather than an intellectual one.
Playing the guitar is more like riding a bike than messing around with a broken clothes drier. Rather than hefty intellectual thought, it requires hours of repetitive practice. Initially, I approached this practice as a thing on it's own, and tried to intellectualize the instrument at the same time. This didn't work. Constantly trying to memorize new things about the instrument and sitting down to rigorous practice everyday took the joy right out of it for me.
I ended up setting the guitar down for some time because of this method of practice.
What I currently do:
Now, I essentially just go with the flow. Half of the time, my "practice" revolves around simple experimentation. When I hit a roadblock with this I do some legitimate, technical "practice" by doing the following:
Pick up laptop
Turn on TV show
Pick up guitar
Mute guitar strings (or not, if you feel like pissing off your roommates/neighbors)
Practice whatever I need to practice while mindlessly watching TV.
Seriously. It's that easy. Just occasionally check to see if you're doing it right, and, eventually, muscle memory will take over. Your desire for experimentation will eventually lead to technical prowess, because, to properly experiment with any instrument, you have to be able to play it up and down.
Tips for a good practice session/tips for maintaining your playing ability:
1. Don't fuck up your technique! Especially when just starting out. If you mess up your technique it could take weeks or even a month or two to set it straight. Muscle memory can be very hard to reprogram.
2. Don't schedule! Seriously, as an INTP (I assume you guys have a similar disdain for scheduling), don't schedule. It'll just drain the fun out of it. Instead of scheduling, just do the above practice that I mentioned; mindlessly drilling in front of a TV.
3. Do sweat the small stuff! Back to technique, something as small as your thumb position can seriously mess up your ability to play. Barre chords can be a serious issue if you don't position your finger right.
4. Start slow! Again, technique, muscle memory, etc. I suggest getting a metronome or something similar to establish a sense of rhythm.
5. Watch Youtube videos (I'll post some cool channels at the bottom), learn tabs to your favorite movie/videogame soundtracks, try to figure out challenging chords, and, overall, just enjoy the hobby.
Oh, and, don't be afraid to set it down if you get frustrated. I did, and the fact that I have a guitar just floating around encourages me to always pick it back up.
Justin Guitar | Free Guitar Lessons
(seriously excellent website)
(there are guitar videos somewhere in there)
Morrowind Theme Tab (ver 3) by Misc Computer Games @ Ultimate-Guitar.Com
(the version in the comments is much easier and fun to play for a beginner)
Oh, also, get a tuner. Seriously, they run at what, 12 dollars? Really, do it.
This is all assuming that you're a beginner. If you aren't, whatever, here's hoping that someone finds this useful.