The first thing to do in putting together your own 12 bar blues in open D major tuning is to set your guitar tune to open D major. The open string notes are D, A, D, F#, A and D. It is called an open D major because it produces a D major sound when strummed open.
In handling the D major tuning, you have D major as your 1 change; G (at the 5th fret) as your 4 change; and A (at the 7th fret) as your 5 change. This makes up your 1, 4, 5 pattern.
The basic pattern for the blues is the shuffle pattern. It is composed of a 5th and a major 6. The strum for this pattern goes like this: “1 and” on the 5th with the downstroke and “the” on major 6 with the upstroke. Practice this pattern on your 1, 4, 5 keys (D, G, A) to get a feel of the blues in D major tuning.
Also, practice the basic slides. Do riffs on D, G and A, and get a feel on it. Riffs and slides are elements that you have to deal with in slide blues guitar.
After mastering the riffs, the slides, the 1, 4, 5 and the shuffle pattern, you can start putting up your own 12 bar blues combination. Try this variation for starters: 1 bar D, 1 bar G, 2 bars D (shuffle pattern) then mix it up with 4 bars of riff in G, then back to shuffle pattern 2 bars D, 1 bar A, 1 bar G and a closing riff.
12 Bar Blues in Open D major Tuning:
watch 12 bar blues in open D major tuning on youtube