How to use Elmah from UI, DAL and BAL

i used Elmah in mvc and i force Elmah to log the error from try catch in action. suppose i have layer application where i have UI, DAL and BAL. now tell me when error occur in BAL or DAL then how can i instruct elmah to log that?

i add elmah in UI layer. do i need to create a separate project in same solution for elmah and expose some class function which will be called from UI, DAL and BAL?

please guide me how project structure should look like as a result i can log error by elmah from any layer.

discuss with little sample code and example.