Decide what key you're going to play in. Find the primary chords in that key. The primary chords are triads, or three-note chords, based on the first, fourth, and fifth degrees of the scale (or I, IV and V), and are the most important chords in a key. For example, if you choose the key of C, the primary chords would be C, F and G.
Play the harmonic progression as blocked chords. The most basic harmonic form of the 12-bar blues uses only the I,IV and V chords. The first four bars consists of the I chord followed by two bars of the IV chord, two bars of the I chord, two bars of the V chord and two bars of the I chord. Play the basic three-note triad with four beats to a bar.
Continue to play the harmonic progression but add extensions to the chords. These are the upper notes of a chord that are not part of the original triad, such as the seventh, ninth or 11th degree of the scale. For example, if we wanted to add the seventh to the five chord, we would count seven degrees from the fifth of the scale.
Add additional chords to the basic 12-bar blues pattern. A common example is using the V chord or the V7 chord at the end of each chorus; this is called a turn around. Another common variant is to change the I chord into a IV chord at bar two.
Play the bass line with your left hand. With the 12 bars there are many types of bass lines, such as walking single note or boogie-woogie.
Add your right hand. If you're a beginner pianist, start with a walking single note bass line in the left hand and gradually add the right hand to it. The more complicated boogie-woogie bass lines require careful coordination between the hands.