Ha! Ha! Not that I know what "12-bar blues" are ...
But I really must like them!
OK, in general, you have 12 bars that comprise a "section" of the music. This is in 4/4 time, which means when you hear the chart (after the giggle and after the drums), you count 1-2-3-4 on the beat. Do that 12 times and you have a "section".
Within that section, you'll have 4 bars of 1-2-3-4 in the key of the piece. Let's say it's G major, or the Roman number I.
After those 4 bars, the chord progression modulates to C major, or the Roman numeral IV. After only 2 bars in C major, the chord progression drops back down to G major for two bars.
That's 8 bars total. But we ain't done with the "section" yet.
After those 2 bars of G major, the chord progression shifts to D major, or the Roman numeral V. After only 1 bar of D major, the chord progression drops a whole step back down to C major, also for 1 bar.
Now we're up to 10 bars of this 12 bar blues. Two more bars to go.
This is where things get really interesting. The progression can go any number of ways, but let's say in this case it goes back to G major for one bar and then the "turnaround" of D major before repeating the entire 12-bar blues pattern.
There's lots I'm leaving out, but this is the general chord progression of 12-bar blues, especially in the Sixties.