Alternating Bass Bum-Ditty Pattern
Now that you have your chord shape down, let's take a look at adding the bum-ditty pattern with alternating bass.
For the D chord, your alternating bass will be on the open 4th string and the open 5th string. Before actually strumming your guitar, practice going back and forth between those notes. Then add in your basic strum. Once you feel completely comfortable with that, move on to adding your bum-ditty. You'll be strumming the bottom three or four strings for this bum ditty pattern.
The first measure in the tab below shows the basic pattern. Practice this first and then loop the second measure to practice the bum-ditty pattern using the D chord.