Watch the Add Muted Bassline online guitar lesson by Vicki Genfan from Acoustic Rhythm Guitar Cookbook
Begin practicing the "palm mute" on the 6th string. Then work on the 5th. Notice that you may need to move your hand down a bit as you attempt to mute the 4th string. You might also want to go through the chords in the progression – without adding any walking basslines, alternate bass notes or hammer-ons – just to put your entire focus on the muting. If you find it difficult to mute the bass notes, and then strum with an "open" strum, then spend a little time just playing the muted bass notes. Once you've spent enough time so that it becomes automatic, then when you add the open strum it won't be so difficult. It's so important to get it right at the beginning, so you don't learn any bad habits. They are difficult to let go of once you learn them!