Theme park to Downtown Disney:
Take a bus (or boat or monorail) to any* resort, change to a bus for Downtown Disney.
* You will prefer to change at a resort that is more or less on the way, for example from Epcot Future World, Port Orleans is a good choice; from Magic Kingdom the Contemporary (walk to it) is a good choice. If you are going to the north end (Marketplace), the Saratoga Springs bus to its Congress Park stop is a very good choice; walk from there.