Hi Kayce! Get to Magical Express is really pretty easy. We flew Southwest and used MME last year. After you get off the plane you are going to head to the shuttles and take it to the main terminal. (My girls call it the mini-monorail). There are lots of signs along the way so no worries there. After a short ride in the shuttle you will be at the main terminal. It's very big. You want to follow the signs to the B side of the terminal. After exiting the shuttle you will go straight and hang a left just past the fountain. Go as far as you can and you should see that you are on the B side. Once you get to the B side take one of the elevators or escalators down to Level 1-essentially you are heading to baggage claim/rental cars they are on the same level as MME. When you get to Level 1 you will be heading to your right and you will run into MME.
At the MME location there will be someone there that will want to see your MME vouchers. Seems like we had to show it a couple of times.

That's when you will get herded into your first WDW queue of the day.

Just keep your documents handy and you will be fine. The map on the inside cover of your MME docs is fairly helpful if you get twisted but I don't think you will have any issues. Don't hesitate to ask someone if you feel like you are going in the wrong direction. As much business as Disney generates at that airport I'm sure they can direct you.
The bus ride really is quite pleasant and the video is a great way to get warmed up for all the fun you are about to have.
Hope that helps a little.
Edited to add: Sorry ladies, I guess we were all typing to Kayce at the same time. Sorry for being redundant.