The first step to following plans to build a bookcase, such as that on popularmechanics.com, is to read through all the instructions and steps carefully. Also check the materials and tools that are required to complete the project, and collect them together.
The next step is to ensure that there is adequate space to complete the project and that safety equipment is in place. Each step should be followed carefully, with reference made to pictures and diagrams to ensure the process is being completed properly. When preparing to cut materials to size, always double check measurements, and use only one set of measuring units throughout the project. Don't be tempted to skip a step or rush through, particularly when waiting for glue or other fixtures to dry. When positioning shelf cleats, use tape to hold them in place before marking the position to ensure that the cleat stays still; this makes lining up the opposite cleat easier.
Assembly is also easier with the help of a second person; two people can keep the unit still while positioning and attaching shelving, as well as moving and turning the bookcase during construction. Always be aware of where the other person is, especially where her hands are, to ensure that they are not placed close to where a tool is being used or glue that is still wet.
If a problem occurs, return to the previous step in the plan, and retrace the work to uncover and fix the mistakes.