Measure the height and width of the gate between each of the thicker support posts. Depending on the design of your gate, these posts may only be at the ends -- or there may be additional posts spaced along the length of the gate.
Cut a lattice panel to fit the measurements between the support posts. Cut a panel for each section between supports if your gate features multiple support posts. Use a circular saw with a fine-tooth blade for making smooth cuts in the vinyl or PVC lattice.
Position the lattice panels over the gate so each panel rests between the support posts. When positioned correctly, the panels should not protrude beyond the posts, which allows the gate to still slide freely. Use plastic zip-ties to temporarily hold the panels in place on the gate.
Place a U-bracket over the top rail of the gate so the bolt holes on the bracket sit near the corner of the lattice panel. Drill a hole in the lattice for each bracket bolt hole, then bolt the bracket to the lattice with a bolt and nut. Install a bracket on each corner of each lattice panel.
Install additional U-brackets over the wrought-iron posts as necessary to hold the lattice in place securely -- and to prevent it from bubbling or bowing outward. The amount of brackets necessary depends on the size of the gate and lattice panel. Cut the zip-ties off once the lattice is firmly bracketed in place.