You need an electric brake controller installed in the truck. It's a unit that determines how much braking is needed, and sends an appropriate signal. Not all are created equal. Ignore anything that is time-delay based, as they are not measuring the amount of braking needed. With this, you could rear-end somebody before the unit has "time" to ramp up the voltage. You want a "proportional" controller, as these send a signal that is "proportional" to how hard you are actually braking. And the best of the proportional controllers will use non-moving accelerometers, rather than moving pendulems. I highly recommend the Tekonsha "Prodigy" P2 or P3 models. Expect to pay $100-130, but it's a relatively small investment compared to the property you are protecting (Explorer, trailer, and contents).
As far as how to setup, check under the dash for either:
a. electrical socket (probably 4 wire going into a 6-place socket). Could be on the driver or passenger side. If you have this, your truck may also have come with a pigtail to fit into this socket. Splice this harness into the pigtail that comes with the brake controller to make a harness for connection. Or Tekonsha sells brand-specific (Ford, GM, etc) harnesses for plug-n-play (no splicing, butt-connecting).
b. Look for 4 wires taped up under the dash. Splice the brake controller pigtail directly to these wires for a hardwired connection.
c. If you have neither of these, then you probably do not have a factory tow package, and will need to have the vehicle wired for trailer brakes. The only wiring you probably have would be a few wires under the bumper for lights, either bare/taped under the bumper, or terminating in the standard 4-flat connector.
PS - It's great that you want to use those brakes.
Don't let anyone tell you that you don't need to use those trailer brakes. The brakes of any vehicle are only designed to handle up to the GVWR of the truck itself. Even a small aluminum frame trailer will probably weight more than your payload capacity. Even with the truck empty and no passengers, you would probably be overloading the base brakes.