Most M1-newbie attempts at loading go something like this: "Rattle, rattle. Crunch. OWWWW! @#$%!" Maybe this page can help you learn without getting M1 thumb.
Before we begin, though, take a look at this clip:
Notice that the cartridges have holes in the sides; they're DUMMY rounds. Do not, under any circumstances, try to learn how to load an M1 using live ammo unless you are at a range, and even then, be extremely careful about where you're pointing your rifle. If you try this at home with live ammo and end up blowing a hole in your ceiling, don't say I didn't warn you.
Fundamentally, your task is simple. With the bolt locked back, put the clip in place on top of the follower and, using your thumb, push it down into the magazine well until it snaps in at the bottom. The gotcha is that as the clip gets near the bottom, the bolt is released and starts to come forward. Unless you are prepared for this, you'll either jam the clip or smash your thumb. So here it is, step by step:
1) With the bolt locked back, put the clip in place on top of the follower.
2) With the heel of your hand holding the operating rod handle back at all times, push the clip into the rifle with your thumb.
3) When you feel the clip snap into place, quickly rotate your hand back. This simultaneously releases the bolt, and gets your thumb out of the way.
4) It may happen that the bolt won't go all the way forward when you release it. This is common, and is not considered a malfunction. All you have to do is bump the operating rod handle to get the bolt to slam home.
Be sure to bump the handle palm-up as shown. If you're holding your hand palm-down or sideways, a slamfire can dislocate or even break your thumb. (A slamfire with military surplus ammunition is extremely unlikely, but why take the chance?)
5) Make sure the bolt is all the way home. It should look like this:
If it looks something like this:
you need to bump the operating rod handle until the bolt locks. NEVER fire an M1 when the bolt is not locked all the way home. Doing so is almost certain to cause you and your rifle serious damage.
In general you should not try to be gentle when loading your rifle. It was designed to be loaded in a hurry by men who are being shot at, and it can handle almost any treatment you give it. More to the point, if you try to do any of the steps above in slow motion, more likely than not you'll only be paid back with a jam.