Learn the basics of the bum-ditty pattern using one string. Bend your index finger to about a 90 degree angle at both joints. Use the back of the nail to strike Strings 1-4 with a door knocking motion. Try to hold the bend and maintain the shape of your striking finger. Some people use the middle instead of index. Use the thumb to play the 5th string.
Use the focus feature to loop individual measures. When learning an instrument you need to do reps just like lifting weights to build muscle memory and improve.