The basic rule of double thumbing it's just use it as a guitar pick one movement down and one up.
So apply it on scales or arpeggios playing for each note the double thumb up and down.
When you get more confident with it go ahead with the next step that is play arpeggio with string skipping, It does mean play one note with thumb down, the next one with thump up etc etc...
If you need more help contact me.
Cheers. Enrico
YouTube WebSite