North airport exit and left at first light.
Little over a mile is next light, two quick lefts onto 528 west.
(to pay toll, about a dollar, using cash, you need to use what looks like an exit ramp)
At end of 528 take I-4 west (southwest).
Take Epcot exit off I-4, heads northwest.
After going under welcoming arches, take second exit onto Buena Vista (west).
I think it is the 4'th light, Hollywood studios on the left, Boardwalk on the right. (Preceded by light with Victory on the left, nothing on the right.)
If you missed the Epcot exit (continuation of 536 west) and took the next exit (Osceola Pkwy) you would be going around three sides of a rectangle where Buena Vista was the fourth side.
The original entrance (which is still the largest) is still called Main Gate. If you missed both the Epcot exit and the Osceola exit you would reach 192 (Irlo Bronson) west and run right into Main Gate taking you onto World Drive which runs north-south. For most guests coming from the airport or downtown Orlando, the Epcot exit is shortest including going to Magic Kingdom.
If you did not come in that way you might want to go out Main Gate at a later date just to see its welcoming arches. South on World Drive, exit WDW, turn around at the first light and come back. World Drive intersects Buena Vista west of Boardwalk.