Watch the I IV ii V Etude Level 3 online guitar lesson by Vicki Genfan from Open Tuning Handbook: Rhythm
It may take some practice to get accurate at hitting only the bass string. Remember to slow it down until you get it clean. Also remember to use your left hand thumb to mute the 6th string while you're playing the first chord (D). Try alternate picking and only down – see which one is better for you. Ultimately, alternate picking will allow you to play faster.