How do I build a storage in between studs?
You could cut the Sheetrock out to the studs. Add a board at the bottom. Finish the back side of the Sheetrock and the wood. I put contact paper on mine.
Cathy: maybe this will help
Here is my favorite one.... https://www.youtube.com/watch?v=PsrjXGGSqGM&t=257s
What type of items are you planning to store in your new found space?
Here are some other examples of how this floor-saving storage can be used:
Is the wall sheetrocked or open?
This is a pantry, but you could use it for inspiration. https://www.hometalk.com/18155865/pantry-between-the-studs
You need to add wood for shelf’s then cover wood with sheet rock and paint
Cut and demo the drywall so that you can expose both side of the studs.
Cut the 2x4 shelves to fit and have someone hold them in place, while you glue and screw them in from the other side of the stud. (2x) This “blind screwing method” should be fine as long as you don’t store three stacks high of heavy canned goods, side to side.