How to hold the thing without fighting it
Okay. Guitar's in tune, you know the parts, you know why it's going to drift on you anyway. Now let's talk about actually holding this thing.
Most people fight their guitar for the first month without knowing that's what's happening. They think their hand hurts because guitar just hurts, or they think they're bad at this because their arm gets tired in four minutes. Usually it's not that. It's posture. The guitar's fighting you and you don't know it's a fight you can just... stop having.
Sitting with it
Sit in a normal chair, feet flat on the floor. Not a couch. Couches are soft and you sink into them and your whole posture goes sideways, literally.
If you're right-handed, the waist of the guitar (that curve where the body narrows) sits on your right leg. The neck angles up and out to your left, away from your body, not flat and parallel to the floor. If it's lying flat across your lap like a canoe, that's the number one thing that makes your left wrist do weird painful things trying to reach the frets.
Left-handed players, mirror all of this. I'm not going to keep saying it every paragraph, just flip it.
Some people cross a leg and rest the guitar on the raised knee, classical style. That works too. Try both for a week and see which one your body likes. I'd rather you find the one that's comfortable than force the "correct" one, because comfortable is what gets you to pick it up again tomorrow.
The strumming arm
Your right forearm rests on the top edge of the guitar body, near where it curves in. Not your wrist, your forearm, down near the elbow. Your hand should hang loose from there, like it's dangling. If your wrist is locked stiff, you're going to sound stiff, and your hand's going to cramp in twenty minutes.
Let the guitar's body support your arm's weight. You shouldn't be gripping the thing with your bicep to hold it up. If you feel your shoulder creeping up toward your ear, that's a sign you're doing work the guitar should be doing for you.
The fretting hand
This is the one everybody gets wrong first, so pay attention here specifically.
Your left thumb goes behind the neck, roughly opposite your first or second finger, low. Not wrapped over the top like you're gripping a baseball bat. I see this constantly, week one, everybody grips it like a bat because that's what your hand wants to do naturally. The fix is to bring the thumb down and back so your palm isn't touching the neck at all. There should be a gap you could see daylight through, between your palm and the wood.
Why it matters: if your palm is jammed against the neck, your fingers come down flat and mute the strings next to the one you're trying to fret. Clean chord changes and clean notes come from fingers pressing straight down, close to the fret wire, with the palm out of the way.
Fingertips, not finger pads. Curl your fingers like you're holding a small ball. Press with the tip, right behind the metal fret, not on top of it and not way back in the middle of the space between frets.
This is going to hurt a little at first. That's calluses building, and calluses beat talent, that's not me being nice, that's just the actual mechanic of this instrument. Everybody's fingertips are soft on day one. In three or four weeks of daily practice they toughen up and stop being the bottleneck. If it's a sharp pain, stop, that's different from the dull ache of new pressure. Sharp pain means something's wrong with your angle, not that you need to push through it.
The Luke story
I've got a nine-year-old in one of my daytime sessions, Luke, who cannot sit still for more than about four minutes at a stretch. First day I'm doing this whole posture talk and he's basically vibrating in his chair. Within the same afternoon he'd picked out the intro riff to a song he liked, by ear, no chords, just messing around until it sounded right. Wouldn't touch an actual chord shape after that. Still won't, months later.
I let it go. He's playing guitar, on his terms, and he's having a good time doing it. Not every student needs to hold the thing the "correct" way to get something real out of it. My job is to make sure the mechanics are available to you, not to force you through them if you've found your own thing that works. Though if you're an adult wanting to play actual songs with chords eventually, you do need this hand position. Luke's the exception, not the rule.
Quick posture check, do this now
Guitar on your lap, right position. Look down at your left hand on any string, anywhere on the neck. Can you see space between your palm and the back of the neck? Is your thumb low, not hooked over the top? Is your wrist relatively straight, not bent at a hard angle?
If yes to all three, you're set up right. If no, adjust before you play a single note. Bad habits here get baked in fast and they're annoying to unlearn later. I know from watching my own hand do it wrong for a couple weeks before somebody pointed it out to me.
Before next time
Ten minutes a day, same time if you can manage it. Just sit with the guitar in position, tune it, and fret a few notes up and down the low E string checking your hand shape each time. Don't worry about sounding musical yet. That's not the point of this week.