429 Auto Pilot
Trim On/ ATT On/ AutoPilot CPL to FD/ Proc loaded (possibly activated depending on what you are trying to do. IE Vectors you would not activate with FMS in the roll mode unless you want to turn direct to the FAF) in GPS with correct airport/ L4 key on DU with FMS 1 or 2 selected. Nav selected on the CHFD then when the GPS cycles to APR and you are cleared for Approach press the APR button on the CHFD. There is an FMS explaining LNAV approaches in more detail, I think it is FMS 7 or 8. Check it out.... Pretty simple procedure.