Most of the ragtime counterpoint I play uses standard tuning, with an occasional 'dropped D' for Powder Rag, for example. It's difficult to advise someone without being there to see what they can already do - certainly I would never advise my guitar students to learn the thumb part first, any more than I would advise my piano or organ students to learn one hand at a time (although organists often need to learn the pedal part separately, for technical reasons - heeling and toeing is rarely straightforward!). It is far better to learn both parts simultaneously, but very slowly and in strict rhythm.

There are occasions, of course, where you may need to work out the way a particular phrase hangs together, and playing the melody line or bass line on its own can be useful to hear how it should sound. But the guitarist's left hand is best learned as a series of chord progressions, with the right hand choosing the individual strings...

Analysing what you wish to play, and breaking it up into 'learnable chunks' is the musical approach - play a small section over and over in strict rhythm. Don't let the timing slip when you repeat, either - play it as if there are repeat marks in the score. Gradually increase the speed as you gain confidence - if you make mistakes you are pushing yourself too fast, so SLOW DOWN!

It is essentially a mechanical process which becomes musical as you take control.

My best advice is that, unless you can accurately hear what someone is playing on a recording, it is best to find someone you can watch.