There are a number of different ways in which you can make bookcase shelves, and the approach you take will typically depend on the type of shelves you wish to make and the materials you want to use. If you are trying to replace shelves for a bookcase that have been damaged or lost, for example, then you should try to match the original material used and measure the size to match the frame you have. You might instead be making shelves for a new bookcase you are building, in which case you should make them fit into the design you are using. There are also ways to make bookcase shelves that are not part of a traditional bookcase but are functionally similar.
Bookcase shelves are shelves that fit inside of a bookcase and support the books placed in the bookcase. The type of bookcase you are using for your shelves will greatly determine the approach you should take in constructing your shelves. If you are making shelves to replace bookcase shelves that have been damaged, for example, then you should match the originals as closely as possible. You should be sure to measure the original, or the inside of the bookcase, for the proper size and match the materials used originally as closely as possible.
If you are making bookcase shelves for a new bookcase you are making, then you should match the materials to what you are using in the bookcase. You should measure the inside of the bookcase and make sure your shelf will fit snug within the bookcase. If your shelves are too large, they could warp the shape of your bookcase, while shelves that are too small could bend under the weight of books and break free from the rest of the bookcase. You should also be sure to attach or support your bookcase shelves with brackets that are strong enough to handle the weight of books.
For very long bookcase shelves, you may also need to use support beams in the bookcase. These should also match the color of your bookcase and should be sturdy enough to provide adequate support. You can also make bookshelves without using a bookcase in a number of different ways.
If you do not want to use bookcase shelves, then you can make shelves that attach directly to a wall. When you do this you definitely want strong supports that can bear the load of books, and these should connect directly into studs in the wall using long screws. You can also make bookshelves using bricks and boards; the bricks provide vertical support and the boards lay horizontally to create the shelves. This provides a rustic look that may be appropriate in rooms with a natural aesthetic.