Hey usa4cc,
Let's use your example of B(ass)-D(own)Up-Up-DownUp, and make it an E major chord.
The Bass would be the Low E string by itself, as a downstroke.
All the rest of the strums would typically be strumming on the strings D-G-B-E with those upstrokes and downstrokes. You could also include the A string (which would be a B note, second fret), but in my own experience it's often left out (for a few reasons). It helps create some space / contrast between the low and high notes, and it's also good to save the A string (in this case) for an alternating bass (which would be a later step).
Does that help?
-Carl.
Carl King[br]GuitarTricks Video Director / Producer