How to install the PTO switch on a John Deere L120?

PTO switch doesn't engage so I bought a new one but need directions as to how to replace it.
Do-Cut Services suggested replacing this because of our on-going problem with the mower not running after about 10 minutes! It will mow fine, but starts to slow down to a crawl while cutting the lawn. If the blades are NOT engaged it will travel forward forever. Any suggestions? I am getting very frustrated with this mower. We have had nothing but PROBLEMS since we purchased it 6 years ago!