Attach a flagpole bracket every 4 feet along the top of your existing fence line. Hold the bracket 6 inches down from the top of the fence with the cupped indention facing up, and screw it to the plank with a power drill.
Place a 4-foot long wooden dowel in each flagpole bracket. Push the bracket as far down into the cup as possible to prevent the dowel from coming loose while you install wire. The angle of the flagpole bracket tips the fence slightly to the inside, which makes it very difficult for cats to jump or climb over the fence.
Rest a ladder against the fence, and climb up the ladder until you can reach the top of the wooden dowel. Ask a helper to hand you the end of chicken wire, and hold the wire against the dowel. Attach the wire to the dowel with three zip ties; one at the top, middle and bottom of the dowel.
Climb down the ladder, and move to the second dowel, pulling the wire tightly to prevent gaps. Wire the fence to this dowel, and continue along the fence line until you secured the entire perimeter with the cat fence. Cut away excess chicken wire with wire cutters.