I don't know if that's going to help someone jumping all the way from 1.9 to 2.6. Possibly the 1.9 -> 2.6 changes are as big as the changes from Standard to Clean.
Some useful questions might be:
Have you built any other Moodle 2.X themes, or just Moodle 1.X?
Are you familiar with Bootstrap?
What level of customisation are you aiming for? Just adding a logo and changing a few colors, or trying to match an existing look?
There's quite a lot of info in the Wiki, but it's not always well organised.
It's hard to know where to start without more info but, for example, are you aware that most small customisations can be done simply by pasting the
CSS into the theme settings?