Sixteenth note strumming is the next natural step in learning strumming after eighth notes. Make sure you've checked out the lesson "Strumming Eighth Notes" before continuing on. Sixteenth notes are twice as fast as eighth notes. You play two sixteenth notes for every eighth note duration or four sixteenth notes for every quarter note. In a 4/4 measure you can play or fit 16 notes in each measure (hence the name).
Eighth notes are counted as 1 - and - 2 - and - 3 - and - 4 - and. Sixteenth notes can be counted as 1 - e - and - a - 2 - e - and - a - 3 - e - and - a - 4 - e - and - a. If the tempo is fast though counting sixteenth notes becomes difficult due to how fast they move.
The rules for strumming sixteenth notes are pretty much exactly the same as the eighth note strumming rules with one exception. Downstrokes are now on the beat and the "and". Upstrokes are on the "e" and the "a" offbeats.
This example shows an E minor chord played as a sixteenth note strum. The strumming alternates between down and up. As you play follow the sixteenth note counting pattern.
Here's a D open string chord strummed as sixteenth notes using the alternate down and up stroke notation.
Sixteenth note strumming gives you a lot of options to create interesting strum patterns. You can now mix in quarter note, eighth note and sixteenth note subdivision strums anywhere in your measure. This example shows a variation of strumming rhythms. Notice how the down and up strokes continue to follow the rules.
Take some time and try different rhythms and rests within a sixteenth note feel. There are many combinations.