Actually, it will be a little different depending on your direction on I-4.
If your are traveling on I-4 from the west (traveling east) I would say exit when you see the signs for WDW - Magic Kingdom. Enter through the main gate (no cost if you have reservations). As soon as you go through the MK vehicle toll gate be ready to turn right. Take the first right (Vista Blvd) and follow the signs.
Now, if your are west bound on I-4 (comming from the east, i.e. I-95) you would want to exit on SR 436 Vineland Road North (this would be a right turn at the end of the exit ramp). Move to the left lane as you will be turning left onto Hotel Plaza Blvd (I think it's the second light). Continue west on Hotel Plaza Blvd to East Buena Vista Blvd (It's a T intersection, you will be turning right).
North on Buena Vista Blvd. Now there are a couple of ways to go but I usually turn left on Community Drive so I can look at the old Treehouse resort. West on Community to Bonnett Creek.
Right on Bonnett Creek and go north to Vista Blvd. Vista Blvd is the road FW is on so keep looking to the right. When you pass a guard shack in the middle of the road you are just about ready to turn right into FW.
The worst that can happen is you will drive around the resort area a little looking at the sights. Generally if you see a sigh that directs you to a "Magic Kingdom Resort" you are going in the right direction.
If you get to the AK or All Stars you made an oops
Good luck