Since hammer ons and pull offs are similar techniques (but playing notes in a reverse order), they are often played together, one after the other.  If you’re new to using hammer ons and pull offs then keep it simple…  Let’s focus on the 2nd fret of any string (just an example).  Now, (1) play the open string, (2) hammer on the 2nd fret of that string and then (3) pull off from the 2nd fret (the same fret you hammered on).  If you do this correctly then you only need to pick the string once, but you’ll play 3 notes.