Originally Posted by: usa4ccI've been wondering how to do strumming patterns for 16th notes.[/quote]

16th notes are simply one possible rhythmic subdivision. They can be strummed in a variety of ways depending on the song & what kind of pattern you want or the song requires.

Anders does a great job explaining counting 1/16th notes & showing some basic strumming options using them.

https://www.guitartricks.com/lesson.php?input=29256&s_id=2486

Originally Posted by: usa4ccIt looks like you do 1 a & e, 2 a & e, 3 a & e, 4 a & e. And then all the ands are also downstrokes, rather than upstrokes.[/quote]

Yes, but that assumes you are trying to strum every single 1/16th note subdivision. Which can be done, but rarely is done throughout an entire song.

Originally Posted by: usa4ccHere's my question. How do I implement that and have good timing?[/quote]

A lot of practice. :) Watch the Anders tutorial & practice a lot.

[quote=usa4cc]And is the physical aspect of it just muscle memory?

Yes.

[quote=usa4cc]What do the ' and " mean?

"and" and "&" are equivalent.

1 e & a

is the same as

1 eee and a

is the same as

1 d & d

Just different ways of writing out the same thing.

[quote=usa4cc]Does that mean 1 and 2 are accented and 3 is especially accented?

No. There is no way to know which strum or note is accented until or unless you see the music notation of the specific song.

The only thing being discussed here so far is how to count. Not how to play anything specific.