Before you begin, determine whether you need to replace the entire mesh or if you can patch the hole while leaving the original mesh in place.

If the hole is larger than an envelope and/or the old mesh is exceedingly jagged or rusty, you should probably replace all of it.

Measure the area you are replacing to determine what size steel mesh sheet you need to buy.

Add a few inches to your measurement on each side to ensure you will have enough mesh.