Sand the wood (start with 80–120 grit, finish with 180–220 grit)
Remove dust with a vacuum and tack cloth
Choose the stain type (oil-based, water-based, gel)...
Measure the space where the bookshelf will go (height, width, depth)
Choose the bookshelf design (number of shelves, open vs. enclosed sides/back)
Select materials (plywood/boards, back...