Install a level string around the trailer, install a wood base, dig a trench in front of the wood base, cut and mount the J-channel and the skirting, and fill the trench to install metal skirting on a trailer. Add ventilation and an access panel as needed.
Hammer a piece of rebar into the ground at each corner of the trailer using a plumb-bob hung from the corner of the trailer to ensure that each piece is vertically level. Tie a string to the rebar, and run it all the way around the trailer about 2 inches above the ground.
Arrange a line of pressure-treated 2-by-4-inch boards in line with the string around the trailer. Drive wooden stakes into the ground behind the lumber, and screw them to the 2-by-4s about every 4 to 6 feet. In front of the lumber line, dig a trench about 6 inches deep, and fill the bottom 2 inches with pea gravel.
Measure and mark a level line around the trailer 4 inches up from its bottom edge, and install the J-channel along the line. Measure from the J-channel to the pea gravel, and cut each sheet of skirting appropriately. Screw the skirting to the lumber and the J-channel, making sure to overlap each piece of skirting with the next and screwing them together as well. Repeat the procedure around the home, and fill the trench with pea gravel. Install at least one vent per corner and an access panel.