It's really easy from Epcot. The exit road takes you out to SR536. You follow that straight about 3-4 miles (don't take the I-4 exit!), go straight through two sets of lights and it will turn into 417 (The Greenway). That's a toll road ( two $1.00 tolls) that will take you to the airport. Just follow the signs.
The drive itself is about 30 minutes. Depending on if you plan to curbside check luggage and if you have to return the car offsite, I would leave Epcot by 2:30-3:00.