Unless otherwise stated, it is assumed that you will always sew with right (pretty) sides of the fabric together. As to your question about which patch goes on the top, it doesn't matter. So you can sew with the triangle on top, or the square on top. I usually put the larger one on the bottom, so I would put the square on the bottom. As mentioned you sew opposite sides, PRESS, then sew the other 2.
This patch is commonly called a square in a square (SIS). There are many you tube videos on how to construct. Here's one:
https://www.youtube.com/watch?v=QsuBvoYHWRk