Choose a simple chord progression. For example, A, D, A and E. Each is played for a full bar in 4/4 time. Use the "open" forms of the chords.
Play through the chord progression once or twice slowly using a down-stroke strum. "Down-stroke" means you strum the strings from the lowest to the highest. Become accustomed to changing between chords as rapidly as possible. Once you are able to change chords without mistakes, you are ready to change your strumming pattern.
Play the "boom" part of the boom strumming pattern by stroking only the root note of the chord. The "root note" is the lowest note of the chord and gives each chord its name. For example, the root of an A chord is A. In "open" form A, this note is played by stroking the second string without any fretting. Play the "boom" note on the first beat of a measure. If it helps, tap your foot or turn on a metronome to create a consistent beat to work with.
Strum the rest of the A chord in the second beat of the measure using a down-stroke. This is the "chick" part of the strumming pattern. After doing this, stroke the root note again on the third beat, and then strum the chord again on the fourth. This produces the distinctive "boom-chick" sound of boom strumming.
Switch to the next chord, D, at the beginning of the next measure. Strum the root note, which in the case of open D is the unfretted third string, and then strum the rest of the chord on the second beat. Strum this pattern again to complete the measure, and then switch back to the open A chord for one measure. Switch to the final chord, E, on the last measure and strum the same pattern, first strumming the root note on the first beat, and then the rest of the chord on the second.