Completing the garment

How do I sew the pieces together? Right or wrong side. Explain the mattress stitch to me if you can. How about other suggestions.:knitting:

Mattress stitch is done with the work side by side right side up.
Here’s a couple pages that might help. Scroll down to finishing techniques here:

Another couple sites that have good instructions.

Here’s a good YouTube video by Norah Gaughan: