Place the box base-down on a flat surface. Place the tape measure inside the box and measure along the inside edges, from base to top, and then from side to side. This will give you the measurements for the shelf.
Using the tape measure as a guide, draw the correct shelf size on the sheet of chipboard and cover those lines with strips of masking tape. This will help prevent the board chipping or splitting when you saw the chipboard. Redraw the shelf size over the tape so you have an exact outline and use the saw to cut out the shelf size. Be as exact as possible if the shelf is too big, it won't fit in the cabinet and if it is too small it will slip down.
Paint the inside and outside of the box and both sides of the shelf with an oil-based paint and leave to dry. Repeat with a second coat and again leave to dry. Add a final coat of varnish.
Insert the shelf inside the box to make sure it fits tightly inside and the remove. Cover three out of the four edges of the shelf with craft glue (leaving only the edge that will be facing out of the box unglued) and insert into the box at the height you wish. Place a pile of books underneath the shelf till it is resting on them and leave to dry. Remove books
Measure the box along one edge to find the exact heigh of the shelf and mark this point on both sides of the box. Draw a line at this height using the chalk along all three sides of the box. Use the nail gun to nail along this line at 2- to 3-inch intervals to further secure the shelf in place. Wipe off the chalk line. Do not worry if the nails can be seen as they will be covered when you decorate the cabinet.
Brush craft glue onto the beads, buttons, or shells you are using to decorate the cabinet and press them on the box until they start to fix. Cover any nails and also along the edges of the front of the box and shelf to disguise any rough surfaces. Leave glue to dry. The cabinet is now ready to display your curios.