Put on all protective gear.
Cut a heavy duty piece of 12/3 extension cord to a length of about 6 feet with wire cutters. Or, you may buy a piece of extension cord cable from the local hardware store. Bright orange or yellow works best because those colors are easy to see.
Strip back the extension cord sheathing with the wire strippers. Cut off all sheathing with the wire cutters. This cable will have three wires: one green, one black, and one white. Make sure the extension cord you use has three wires and not two.
Remove the threaded nut from a 1/2-inch romex connector. Insert a romex connector loosely over the end of the extension cord cable. Push the exposed wires you stripped into the bottom of a double-gang plastic box that is waterproof. You want about 16 to 18 inches of the extension cord cable and exposed wire to be inside the box.
Insert the romex nut back over the threaded collar and tighten down with a small pair of pliers. Use a screwdriver to tighten down the two clamp screws that will hold the romex connector tight to the extension cord cable.
Loosen the set screws on two GFCI outlets. Cut about 6 inches off of the extension cord cable that is now inside the box. This is to be used for jumper wires from one GFCI outlet to the other. (That's why you left 16 to 18 inches of extension cord cable in the box.)
Pull the three wires out of the sheathing. Strip their ends with the wire stripper. Run one jumper wire from the gold screw on the first GFCI outlet to the gold screw on the next GFCI outlet. Do the same for the white wire, except you will attach it to the silver wire. Repeat the process with the green wire, connecting it to the green, ground screw.
Attach the extension cord wires to the colored terminal on the second GFCI outlet. The jumper wires from the first GFCI outlet will already be attached. Connect black to gold, white to silver and green to green, just like you did with the jumpers.
Push the GFCI outlets into the double-gang box and screw them into the box with a screwdriver. Line up the screws on the GFCI outlets with the screw holes on the box.
Screw on a waterproof cover over the plastic box.
Strip the opposite end of the 6-foot run of extension cord cable, just like you did for the other end that went to the box. Remove the cover from a plug end. This plug end will attach to the extension cord cable and allow you to plug in to a power source. Use a screwdriver to remove the cover.
Slide the stripped wires inside the plug end. Attach them to the colored screws, just like you did on the GFCI plug. Tighten with a screwdriver. Replace the cover.
Test the new portable outlet by plugging one end into the wall and plugging in some electrical device to the GFCI box you made.