You should have a total of eight pieces: four sides, the lid, inner lid panel, handle and the bottom (with my seal and copyright).
Inspect all of the parts for problems with the material. Whether plywood or MDF, my experience is that the parts are sticky, especially the edges, and I usually wait a day until the sap/resins dry.
Clean the edges of all the pieces with a Shop Cloth or Paper Towel dabbed in Orange Hand Cleaner. Any burn marks on the surface will be removed when you sand the parts.
The Hand Cleaner will leave some residue behind, so blot it off with a damp cloth or paper towel so the residue will not interfere with the finish later. Don't use too wet a cloth or it will cause the wood grain/MDF to swell and possibly warp your parts.