Watch the 12 Bar Blues in E Part 1 online guitar lesson by Brad Carlton from Blues Bass
Look at the rhythm chart number one and you will see that the 12 bar blues progression in the key of E involves three different chords: E7 which is the I7, A7 which is the IV7, and B7 which is the V7. It is important to memorize this progression in terms of letter names of the chords as well as the numerical labeling. To help you memorize this progression, I suggest you view it as three separate four bar sections. The first four measures are the I7, the second section is two measures of the IV7 and two measures of the I7, and the third section is one measure of the V7, one measure of the IV7, and two measures of the I7. You will notice that the 12th measure has the B7(V7) in parentheses. This is an optional chord change which is very common.
Your goal is to take all of the basslines that you learned in the previous lesson and play them over this jam track. Once you can do this you can start experimenting with mixing different patterns in different measures.