Most have problems with strumming right at first. It's a feel thing, and I always hated it when the old guys told me to feel it. So..... here are some hints.
1. The vocalist will not like it if you switch back and forth between strum patterns during a song. Pick one and stay with it - for the entire song.
2. Nothing wrong with all down strums - it's like riding the root, there are better things, but until you get the feel all down strums get the job done.
3. Boom, strum, strum, strum. Boom is a pick of the lowest string used in the chord followed with three down strums - 4/4 time of course.
4. Boom, strum, boom, strum. The second boom is on the next to lowest string. Watch how Hank does it.
Now how do you get a strum pattern into muscle memory? Go watch TV, take your guitar along and grab the neck around the 5th fret - you will get a dead thud sound - not a problem. Now do your strum pattern while watching TV. After an hour's program that strum pattern will be in muscle memory and belong to you.
It's the old P word. Good luck.