To get to Fantasia from a WDW hotel, take the bus to any theme park and then get on the Boardwalk bus. This is also the bus going to the Swan/Dolphin. Get off at the Swan/Dolphin and walk to the back of the resort where the pool area is. You will then walk to the road and cross the street. Fantasia is right there.
When I did this walk, I was at the Boardwalk on the boardwalk. From there you just walk to the left, over the bridge to the Swan/Dolphin and keep walking relatively straight(past the pool) until you get to the street. Fantasia is on the other side.
To get back, you would need to walk to the Swan/Dolphin bus area and take the bus back to a park to pick up your resort bus. I think there is only one stop for the Swan/Dolphin, but if I'm wrong someone will post.