This all depends on your TV and whether it supports discrete input commands. If your OEM remote allows you to press one button and go directly to each input then you're all set. Or even if the OEM remote doesn't have them sometimes they are available in the harmony command list - just try any command that looks like an input selection and see what happens.

Each device has an Adjust Inputs link where you tell the harmony how that device selects the inputs. Then, for each activity you tell it which input to use for each device and the harmony figures out how to get there (dynamically).