Measure the distance between the two gate posts and cut a fence panel 1/2-inch narrower than the opening. Standard measure anywhere from 36 to 48 inches wide, and a single gate with a wheel can be constructed up to 60 inches wide without difficulty. Use a circular saw to make the cut. Mark the panel with a chalk line for a smooth, straight line.
Cut a two-by-four piece of treated lumber to fit diagonally between the top of the lower horizontal rail on the hinge side and the upper horizontal rail on the latch side. Lay the uncut two-by-four on the back of the fence panel aligned at the correct angle and mark the intersecting points between the two-by-four and the horizontal rails, one inside and one outside at top and bottom ends. Cut between the points for a perfect angle. Screw the support in place through its angled ends into the rails using 3-inch treated deck screws.
Lay the gate face up on a pair of saw horses. Position the hinges in line with the center of the top and bottom horizontal rails, with the hinge pin centered on the hinge side edge of the gate. Mark the screw holes and drill pilot holes using a bit the same diameter as your hinge bolts. Attach the hinges with one carriage bolt in each hole, using a fender washer and nut on the inside of the gate. Tighten the nuts with a socket wrench.
Check the bottom edge of your fence. If it is resting at grade, or on the ground, mark and trim 1/2-inch from the bottom of the gate. If it is suspended above grade, do not cut the gate panel. Measure the distance from the bottom edge to the ground.
Mount a block of 2-by-6-inch treated lumber reaching from the bottom edge of the lower horizontal rail to the bottom edge of the gate. Attach it with 1 5/8-inch treated deck screws through the face of the panel into the block, and with a 2-inch L bracket in the corner between the block and the lower horizontal rail on the inside edge.
Position a spring loaded adjustable gate wheel on top of this 2-by-6-inch block with the adjustment all the way up to give it the most adjustment. Set the wheel below the bottom edge of the gate equal to the distance you measured from the bottom edge of the nearest fence panel to the ground. Mounting procedure varies from brand to brand, consult the instructions with the wheel for specifics. In general, mark the bolt holes in the wheel's mounting bracket on to the 2-by-6-inch block.
Bore pilot holes in each marked location with a bit equal in size to the wheel mounting bolts and bold the wheel in place, tightening the nuts with a socket wrench.
Stand the gate up, with wood blocks under each end to raise it to the height of the fence and prevent the wheel from rolling. Place it snug against the hinge-side post and mark the bolt holes from the hinges onto the fence panel. Bore pilot holes and attach the hinges as for the gate side.
Install the gate half of the latch first, boring pilot holes and snugging the bolts with a socket wrench. Mark the center of the latch bolt on the latch side post and center the post half of the latch on this mark, boring pilot holes and tightening the bolts with a socket wrench.