Old Man song

For me it was a feel thing.
Being familiar with the song and the sound I was after.
Down strums have a deeper sound because you are playing the bass strings, up strums have a higher sound (only catch the top 3 or so strings.
Knowing that (this is what I did) forget the strumming pattern. go for the sound instead.
PS: after you get comfortable with that first part pay special attention to the D, Dsus4, Dsus2 section. That progression is used in alot of songs so it is good to be able to switch between those D chords fluently.

You really don't have to play it EXACTLY like David has tabbed it out loosen up a bit and have fun with it. As long as you hit a few of ther strings in the chord without catching the bass strings it should sound fine.

Basically David is just combinining full and partial chording with some single notes. If you listen to the recording it is quite a bit different, but it has the same feel.

I suggest you try to strumm in time to Davids using full chords. When you get that down just start feeling out the sounds of single note and partial chords replacing the full chords in different places.

I agree with everyone. Example #1 is different from Young's recording. I remember that lesson. I think I did it a dozen times (you know, about 5 minutes) before I was off doing it by feel and sound. I'm still off form the recording. Young does a "partial palm mute", however the heck you do that without pulling a muscle.