I want finished seams on the inside

Put the wrong sides together and sew seams so it looks like the seams are on the outside of the bag. Turn the bag inside out and sew slightly inside the bulky seams you'll now have along the sides. When you turn it back to the right side - the seams are covered.

You do it like two pillocases stitched together with an opening for turning. Turn right side out, push the lining to the inside and then handstitch opening closed. (hope that makes sense)

Sorry this won't make sense if you're doing a lining instead of a 1 piece. I make purses with pre-quilted fabric so this is how I hide the seams.