If you are using MDF, either purchase shelf boards, or MDF in strips 11 1/2″ wide (could be a different width, but this will change the depth of your bookcase) but 3/4″ thick, or have 3/4″ MDF cut into 11 1/2″ wide strips at the lumber store. If you are using MDF, use 2″ drywall screws to fasten the box together, predrilling and countersinking your screws correctly. You can use finish nails to fasten the frame to the face of the bookshelf. For a better fit, you may wish to measure and cut your trim to fit the shelf as you are building. Work on a clean level surface. Check for square after each step. Keep outside edges flush. Use proper safety equipment and precautions. Respect the saws, watch out for stray nails!