Prep the area by laying down tarps along the edges of the fence and over any plants nearby. If the fence is close to a building, close all windows in the area near it, and tape newspaper over them if you're using a spray material at any point in the process. If the fence is attached to the building, tape newspaper over the walls near the fence to protect them.
Remove any rust spots or loose paint on the fence. If there are few spots that need to be taken care of, and they're small, you can remove them with a wire brush. For larger areas, use a sanding disk attached to a rotary tool to speed up the process.
Wash the fence with a sponge, using a solution of 1 part bleach to 4 parts water. Allow the fence to dry.
Apply a rustproofing agent to any rust spots on the fence and allow it to dry. This helps prevent the rust from spreading. Depending on which type of rustproofer you have, you may either spray it on or apply it with a brush.
Apply a coat of primer to the fence. If the opening between the wires are fairly small, you can use a paint roller to apply the primer, but if the openings are more than 1/2 inch apart, use a paint brush, and make sure you coat the inner edges of the wires with the primer. Prime one side of the fence at a time, and start from the top corner and work your way down the first section before starting at the top of the next section. Use a primer that is specially formulated for galvanized fencing. Allow the primer to dry.
Paint the fence, using the same technique you used when applying the primer. Ask for assistance at your local hardware store regarding which type of paint will work best in your location. For instance, if you live in a hot, dry area, an acrylic paint would work well, but you'd be better off going with a urethane paint if you live in an area that gets a lot of rain. Allow the paint to dry.
Add a second coat of paint, or touch-up areas you missed with the first coat, and allow it to dry.