There is an excellent guide over on MSDN on how to debug a SharePoint timer job from Visual Studio. However, it fails to mention that sometimes you need to restart the service that runs the timer jobs to allow debugger to set a breakpoint. In addition, sometimes a job will not show up in the job definitions list (Central Administration – Job Definitions), making it hard to manually start a job. The solution to this is to recycle IIS.

The simplest way to incorporate this into a development workflow is to add it to these commands to the Pre- and Post-deployment Command Line steps in Visual Studio:

Pre-deployment Command Line:

net stop SPTimerV4

Post-deployment Command Line:

net start SPTimerV4

Hope this helps.