I would practice just going from one chord to another and back again with the simplest strum and work up from there, still using just 2 chords.
One of the easiest strums is all downward strokes on quarter notes.
If you don't know what quarter notes are, I can explain it like this:
Most music is in what's called "4/4" time. "4/4" is a "time signature." The number before (or above) the slash tells you how many beats there are to a "measure." A measure is what you see separated by straight lines in sheet music. The number after the slash tells you what kind of note counts as one beat.
So "4/4" time means that there are 4 beats to a measure, and a quarter note counts as one beat. Using the same logic, we can see that "6/8" time (another common time signature) means there are 6 beats to a measure, and an eighth note (half the length of a quarter note) counts as one beat.
Lengthy explanation aside, start with two easy chords: C and F. We'll use "4/4" time, and each chord will last for one measure (4 beats).
Count the beats aloud as you play like this:
"1 and 2 and 3 and 4 and"
Finger your C chord. Strum down when you say a number. When you say "and," your hand comes up but does not strum.
When you get to the last "and" after the 4th beat, switch to the F chord and do the same thing, this time switching from F to C on the last "and".
Keep doing that for a few minutes until you can switch from chord to chord without missing a beat. There should be no lag time while you switch chords. If you need to go slower, go right ahead.
Once you've got that going pretty good, we'll add an up stroke.
This time, say the following "mantra" out loud as you strum:
"1 and 2 up 3 and 4 and"
As you can probably guess, you'll strum up when you say, "up".
On every "and" your hand is coming up but not strumming. When you say "up" after the 2nd beat, your hand will touch the strings and strum.
If we were to replace the numbers in our "mantra" with words, we can say:
"down and down up down and down and"
If it's easier for you to strum with this "mantra," go right ahead.
Let yourself get comfortable with this new wrinkle in your strum before we add another. Give yourself as long as it takes. Remember, you shouldn't miss any beats! There should be no break in your speech cadence as your switch chords.
Ready for another wrinkle? Try this one:
"down and down up down up down and"
Now that we have this method of constructing strums, you should be able to explore and create your own.
