Watch the The Basic Blues Form online guitar lesson by Jeff McErlain from Blues Survival Guide: Rhythm Edition
Here is the basic 12 bar blues form. I know this is easy, but please don't sell it short. I can't tell you how many times I have encountered people messing up the form on a blues. This is the equivalent of having a rug pulled out from underneath you and on a gig is pretty bad. The band, singer, and soloist are counting on you to hit the right chord at the right time, if you mess it up your chances of getting a call back are slim! I highly recommend practicing with recordings, the enclosed jam tracks, and solo with a metronome. I know this seems easy but I have messed it up myself on gigs and I didn't get called back! You need to internalize the form so you feel it, not think abut it.