I suppose I could retype what page 37 in the manual states, it states it rather clearly there. After you put the CC(s) in, refer to page 57 and turn CC only mode ON. You must know which PLAN the system you want to monitor fits under. You can find this out by looking at the frequencies in the system and matching it up with the most appropriate type listed on page 57.
No you don't need to clear the scanner, but you'll be confused if more than one active frequency is programmed in the same bank you are programming the trunked system in, unless you are familiar with the 780.