Please help... how do I seam the bound off / cast on edge on a garter knit square?

[DowntonIsMyHome]

I can do the two edges of my cushion, but I can't figure out how to turn the seam "inside" for the top and bottom (like mattress stitch does on the edges)

Please help!

I think I would pick up the outside edge of every cast off stitch so that the inside one stays inside and what you see is the first/last row. Stitch right side out. Not quite sure what you mean, but does this make sense?

yes thank you I just couldn't figure it out !