The Sonobe unit
is a pretty common modular origami
unit. This instructable will show you how to make the basic Sonobe unit. There are several photos of each fold, so look at the pictures. The text is just a basic overview; the information is in the pictures.
If you have questions or need extra pictures just add a comment.
There are ways to do this in fewer folds, but the extra creasing steps are helpful for beginners.