The easiest time to sand the inside corner of a small box is before it becomes a corner. Once the sides and dividers are together, there is less room for a sanding block and more chance of scuffing the neighbouring face. A little work while the parts are flat saves an awkward reach later.
Dry-fit the box first. Decide which faces will point inward, then mark their orientation lightly in pencil where the mark can be removed or hidden. Note the surfaces that will meet at each joint. Sanding should improve the exposed faces, not change the dimensions of a carefully fitted slot or shoulder.
Support each part on a clean, flat surface and secure it so it cannot slide. Use a flat sanding block for broad faces, starting with a grit suited to the marks you actually need to remove. Work through the appropriate grit sequence for the planned finish, following its instructions. Use dust extraction where available and suitable eye and respiratory protection.
Keep the block flat as you approach an edge that forms a joint. Rolling over that edge can leave a visible gap even when the parts fit well elsewhere. On thin plywood, sand lightly: the outer veneer is limited, and sanding through it is not something another coat of finish will hide.
Remove the dust with a vacuum and a clean brush, then dry-fit again. Check that the orientation marks still make sense and the joints close as before. Pre-sanding does not require pre-finishing; if you also choose to apply finish before assembly, keep glue-contact surfaces bare unless the adhesive maker specifically allows otherwise.
Practical takeaway: add one pause between the first dry-fit and the glue-up. Lay out the parts, smooth the faces you will struggle to reach later, and test the fit again. Leave final exterior sanding until after assembly, when you can deal with the box as a whole.
SandingBox MakingShop Tips