The 12 different measures are something that makes it hard for you to learn the 12-bar blues. You must observe the particular scheme and theme of things in this sort of music playing. Always notice that in such form of playing, there are no letters involved for chords; in fact, there are Roman numbers involved. These Roman numbers will help you understand the progression of every key, which is also going to be the same. For instance, if you are playing blues in the progression of E, then the I chord is used. Same is the case with A as it will depict the IV chord. Moreover, you play B as a bar chord, which is also later explained.
You start off by the quick four. It is a variation in the 12-bar blues as the second bar is always different in this case. You should understand that rather than playing the I chord and staying on it for at least four measures, you move to a IV chord. In case of A, you start playing it in the key of E. Therefore, it always becomes difficult for guitarists to keep switching between keys while sticking to one. It will give you the option to add variation to your guitar playing. The same chords would sound broken on any other platform.
The turnaround will help you add progression to the last two bars which will take your music back to the beginning. Either you can repeat the progression after you are done with the first 12-bar blues, or you can always end it the way it has ended. Always use a turnaround which will set up the repeat. Mostly, people change the V chord with the I chord. They believe it is the most common way to do this in the last measure for a turnaround.