How do I read a knitting gauge?

[davina1]

I'm starting a new project and i'm trying to be good and determing my stitches per inch before I start knitting.

So, I bought this Boye knitting gauge with holes to determine your needle size, some measurement marks that make no sense to me in the middle, and a sliding part at the bottom that says "stitches per inch."

No instructions included. How do I use this thing to measure sts per inch in garter stitch.