Continue writing the next sentence.

How to install an aftermarket headunit in a ford explorer
Research head units and pick the one you like the best.
Important features are watts, pre-amp outputs (if you want to hook up new amps later), direct ipod connection, and there are many more.
Buy the unit.
Determine if your explorer has the jbl audio system or if it not upgraded. If you have jbl, you will have more work to do, but the end product will sound better.