I use old sheets, solid light colored fabric or muslin for the foundations, this fabric can be light-weight and see through.
Cut your foundations to about 1" larger than you want the finished square to be. Mine will be 6" in the finished square so I cut mine to 7".
I cut my fabric scraps into strips (or strings) that are between 1 1/2" -3" wide, I leave them however long they happen to be and cut them as I add them to the block. I vary the width of my strings but they can be made all one width, it depends on the effect you want. I also keep the smaller scraps and cut them into triangles for the corners. This keeps me from having tiny little pieces in the corner.
To start building the block, take two strings, place one on the foundation from corner to corner, then place the second one face down on top of it matching up the edges. Sew with a 1/4" seam onto the foundation. Iron flat and add strings, ironing after each one, until it's almost covered. Then add the triangles to the corners. it should look something like the block above.
Then your going to trim the block to your finished size. Since the blocks are going to be 6" in the quilt and I'm going to be using 1/4" seams, I'm trimming this block to 6 1/2". There are a lot of things that can be done with strings for quilts, this is one of the easier ways I've found to do it. The blocks can be set any number of ways to make a variety of quilts.
Here's some links to some of my (unfinished) string quilts.
OK, so I forgot to take a pic of that last one before I sent it off. LOL