Most of the work in a shuffle is actually done by your strumming hand. Rest the side of your strumming hand just in front of the bridge of the guitar. This is a technique called "palm muting" and it will help to dull the sound and give it a thick, gritty feel. The other thing you need to practice is trying to pick only two strings at a time - the one that is being fretted and the one above it. This can take time and practice, but I'm sure you will get it!
In the video below, he uses a variety of chords, but you don't need to worry about those, just remember the 2-4-2-4 fretting pattern and do your 4-counts that we practiced in the first song. Use the following string pattern as you go.
A - 2 bars (8 counts)
D - 2 bars (8 counts)
B - 1 bar (4 counts)
A - 1 bar (4 counts)
Complete this pattern twice through, and you've played a new variation of the 12-Bar Blues!