Have you looked at this page?
There is a general description of the matrix format along with two examples.
If you are defining boundary conditions in a MATLAB script, my own preference (and recommendation) is to write a "boundary file" (user-written MATLAB function) to define your boundary conditions instead of trying to create a boundary matrix. I find writing a boundary condition function to be much more straightforward than writing a boundary matrix. The approach is documented here:
Bill
