If you know the notes in the chords you want to play you could use a fretboard chart that shows which notes are where in DADGAD. I tried to post one but the spacing went wrong and I can't remember how to set a mono spaced font on here. If someone can remind me how to do it I'll post the chart. However knowing the names of notes is not essential, you can experiment.

People who make up their own chords might experiment with bits of chord shapes from other tunings. If they like the sound of what they get they use it and if not they try again. Part of the charm in alternate tunings is in the use of open strings with the fretted strings. Look at the open strings. You will see a couple of examples of octaves on non adjacent strings. These octaves continue as you go up the fretboard. You can fret the same fret on the 6th and 4th strings and and get two notes an octave apart. Experiment with that and playing some open strings at the same time.

Play the 3rd fret on the second string and the 4th fret on the fourth string with the open strings and you get D7. Slide them both up two frets and you get D add2 or, as some people call it D add9. In some places playing both strings at the same fret might sound good. You can experiment and trust your ears if you like what you hear.