Mattress stitch to close up pouch

[Nicolapen]

I crocheted a rectangle and wish to fold in half lengthways and use mattress seam to sew sides and bottom together to create a pouch

Should i start with the right sides on the outside or inside

I found some tutorials of mattress stitch but they were about flat seams

I have included a photo but forum turned it sideways

[bgs]

Its really up to you and how you want it to look.  Try one way if you dont like it take it apart and try another way.