[u][b]Disclaimer [/b][/u]this guide is provided "as is". It's your guitar, so any work you underatake on it must be your responsibility. Make sure you understand the procedure fully before you start any work and please don't practice this type of thing on the pre-CBS Precision Bass you found in the attic.
The following describes how to shim the neck of a Fender-style bass (e.g. a Precision or Jazz) or any bass with a traditional bolt-on neck construction.