The ladder stitch is the best way to sew a seam opening closed. It’s simple and doesn’t take a lot of time.
The ladder stitch step by step:
1. It’s best to sew it up with a single thread. Tie a knot at the end and insert your needle from the inside so that it comes out on the fold (near one end of the opening).
2. Go straight across from where the thread came up and take a little stitch in the other fold. Again go straight across and take another little stitch in the opposite fold. Continue all the way to the end of the opening.
If you sew it precisely and densely enough the stitched place won’t be visible.