Designate the key in which you want to play. Usually this will follow the range of the singer's voice. The key of E, for example, is one that fits male voices well and is easily played on guitar. Know or write down the notes of the scale that you are going to play --- in the key of E, the notes of the major scale are E, F#, G#, A, B, C#, D#, E.
Identify the first, fourth and fifth notes of the key to play a chord of those names. For example, in the key of E, that would be the E chord, the A chord, and the B chord. The E is called the One; the A is the Four; and the B is the Five.
Choose your time signature. Use 4/4 time for true blues. Count out four beats. In music theory, this is a measure. It is also called a "bar."
Play a pattern of 12-bars using the three chords in this format: One, Four, One, One, Four, Four, One, One, Five, Four, One, Five. For example, in the key of E, play a bar of each chord as follows: E, A, E, E, A, A, E, E, B, A, E, B.
Play the pattern through until you are comfortable with it. Sing or play a melody over the pattern the next time through. Use the notes that are found in the major scale for now --- to hone the song, you can incorporate other notes later. This can become the verse.
Make up another melody for the chorus. This usually has a repetitive, easy to remember, sequence of notes in it.
Play two verses and then the chorus, then the last verse and the chorus twice.
Write lyrics for your song --- typically, there is a rhyme at the end of each fourth bar.
Give the song its own character by varying the patterns, adding fills and interesting accompaniments.