Exercise for the Djembe
Many of rhythms from West-Africa can be considered as binary(4/4, 8/8...etc) or ternary(12/8, 3/4...etc) beat.
Here are the practice rhythms for each different pulsation. R marks show the positions you play with the right hand but try to play with opposite hand positions when you can play these rhythms.

4/4 Rhythms
Tap the 8th notes, playing hand to hand.
You can count the notes like this:1234, 2234, 3234, 4234

12/8 Rhythms
The following exercises train your hand to hand playing in 12/8 time. Tap the 8th notes through, evenly.
You can count the notes like this:123, 456, 123, 456

3rd rhythm can be played with ongoing handing but this is more practical. The Basses are played by both hands alternately in 4th rhythm.