To strum, you need to have a motion coming from you elbow, at a steady eight note space.
What gives your strumming pattern their sound is whether you hit the strings coming up or down...

let's take a bar: you have 8 eight notes (nothing unusual here)
let's name them 12345678...
Now, you arm is coming down on 1357 and is coming back up on 2468.
Let's say you're going to play 13567. You need to make it with the same 12345678, but not playing the motions you shouldn't (here, it's the 248) even though you don't play them, your arm DOES the motion! That's pretty complicated to explain on a forum, feel free to ask again...