This is complicated:
From TL opening until about 11:30am, buses run from TL to DTD, but not the other way. Before 11am, there should be Transportation cm's wearing orange safety vests stationed around the bus load zones. Ask them for guidance if you need to get from DTD to TL before 11am; they may be able to get you a direct bus, or at least put you on a bus that will be stopping at TL after going to a resort.
Between 11am and 2pm, buses run from DTD to TL, but not the other way. All buses leaving DTD (except those destined for the MK resorts thru 3/28) should be stopping at TL. To get from TL to DTD between 11:30 & 2, take any bus to a nearby resort (such as SSR or CBR); it will be returning to DTD after the resort, so no transfer is required. Always verify with the driver that the bus will be returning to DTD, especially if it's shortly after 11am.
After 2pm, buses from TL run only to Epcot. (Thru 3/28, buses will also be running to the MK resorts as a limited test.) Since there's no direct service from Epcot to DTD, you would need to transfer twice to get to DTD. This is a situation where the $5-$10 for a taxi for the very short ride to DTD would definitely be worth it. There are usually 1 or 2 cabs at the taxi stand across from the TL park entrance; if not, call 407-422-2222 for Mears Taxi.