Highway 417 is the road you'll want to take. Once you get your rental car, head towards the South Exit to Toll 417. Turn right onto 417 (or just follow the signs to Disney World/Tampa). This road should just lead you straight to WDW. There will be 2 toll roads along the way. Stay in the right lane to access the toll booths. One toll I think was $1.75 and the other one I think was $2.00. We used the Exact Coins booth which is usually to the left at the toll booths. Big Blue sign, can't miss them. We took a roll of quarters with us to use this lane.
After you pay the 2nd toll,
You will see a sign that says "Magic Kingdom, Hollywood Studios, Animal Kingdom, 4 Miles." Immediately after there is another sign for "Exit 6, International Drive. To: 536, 535, I-4 East. 3/4 mile."
A little further down the road you will see another sign that says "Exit 6. World Center Dr. EPCOT, Downtown Disney, Next Right."
Take Exit 6 for International Drive/World Center Drive. Keep going on World Center Drive until you come to 535 North/Kissimmee Vineland Rd. It will be the 2nd light after you exit (1st. light will be International Drive). You'll see the Caribe Royale resort and the Buena Vista Suites Hotel on your way.
Turn right onto 535 North (Buena Vista Suites is your landmark here. You will turn right onto 535 at this hotel.) Keep going on 535/Vineland until you get to Vinings Way Blvd. Turn right onto Vinings Way and Blue Resort will be on your right hand side.
If you get to the Interstate Exit or see this
You've gone too far.
Alternative routes in case you miss the turn onto 535 North.
If you miss 535/Vineland, you can always take Interstate 4 East to Exit 68 (Lake Buena Vista). It will be the very first exit that comes up on the right once you get onto the interstate. Just stay in the right lane. Take Exit 68 then turn left on 535 North.
If you get to the Disney Sign I posted (which I would be tempted to purposely do

), you can always take the first exit on the right after the sign to get back.
Take the first exit on the right (for Typhoon Lagoon, Downtown Disney) after you pass under the sign. Stay in the right lane and merge right onto Buena Vista (Sign for: Typhoon Lagoon, West Side, Pleasure Island, Marketplace). Keep going straight until you get to Hotel Plaza Blvd. Buena Vista will eventually become Hotel Plaza if you keep going straight. You will pass Downtown Disney on your left.
Note: Do not turn left onto Buena Vista at the end of Downtown Disney. Make sure to go straight to merge onto Hotel Plaza Blvd. Best to stay in the right lane. Hotel Plaza will get you back to 535 North. Once you get back to 535, you'll want to turn left onto 535. You'll also be able to use this exit/entrance to get to/from WDW.
If you have a GPS or use GPS on your phone, I would use that to help; AFTER you exit onto World Center Drive. Don't use it from the airport, it will try taking you a different way. At least the one on my phone does. I ignore it until I start getting close to where I need to go and then follow the directions.