This partcular model is so well-known in the paper-folding origami community that it is perhaps the incarnation of modular origami in general. The most widely used folding technique is based on Francis Ow’s 60 degree unit and the arrangement by Thomas C. Hull.
This model consists of five intersecting tetrahedras (FIT) which is also the name of the design. Each tetrahedra is made of 6 edges. 3 edges meet in a corner where they are linked with a flap-pocket system.
There a a lot of sites with instructions on how to fold this well-known modular origami which is sometimes called “spikey ball”. This is one of them: Five_Intersecting_Tetrahedra.pdf. It covers the instructions themselfes, Francis Ow’s 60 degree unit as well as some background information about the mathematics behind it.
Actually this was my first modular origami I folded. And I think about that time I fell in love with this area of origami. There are more models to come which are even harder to assemble. I found out about Robert J. Lang. He also was amazed by this model and wanted to explore other woven polyhedra.
Anyways, here are some pictures of the assembling process. One picture for each new tetrahedra.