Newbie alert! Square slippers pattern question

I’m going to make the square slippers pattern that is listed in the free patterns section of kH. It’s all done in garter stitch (no problem there) but you have to sew the little square together. It doesn’t say how (stop laughing). I watched the mattress stitch video and the others but they don’t really seem like they pertain to this type of work. Help, how do I sew these things together? What stitch?

There are illustrations here that show how to join garter stitch. Hope that helps you out.