Trivial Configuration (for most users)
User has source code placed in multiple subdirectories and either has the current directory to be project root or rarely deletes the tags file that is generated.
First time round, use
:cd to get to the project root and run:
After the first run, you can open any source file in the project with the current directory set anywhere you like; the existing tags file will be found and the location of that will be used as the project root.