All you have to do is hit the note without strumming and make sure that you strum the first note in a 'set.' You don't need to have them all down. It's important to note, however, that hammer ons and pull offs can only be used when the notes are really close together. On settings such as Medium, there aren't many chances to use the hammer ons and pull offs. The notes that you can hammer on/pull off look a little bit different that the regular notes.
For, now, I would just worry about down strumming and hitting the note on time.
P-Worm