How Can I Sew A Rectangular Sleeping Bag Into Something Tighter?

I’m looking to use sturdy thread and either a needle or sewing machine to transform my rectangular bag into one that fits slimly and snugly, with a really tight result. The stitching needs to be secure.

I already have suitable thread and the rectangular bag on hand. Feel free to share images, videos, tutorials, or detailed comments. Appreciate your help.