There are some good instructions for modular origami projects using the sonobe unit out there - for folding the unit this one is good, and for assembly I like this one.
The version of the unit that I like to use is a simpler one than is presented in the instructions that I have found online, and is taken from Origami for the Connoisseur, by Kasahara and Takahama (not really being a connoisseur, it is one of the very few things I can put together from this book). I've attempted some diagrams for this below.
The unit has a few less folds than the standard version, and this makes a difference when you are making 30 or more of them.
The Simple Sonobe Unit
Sonobe Unit Assembly
Update: a handout based on these diagrams is here.