Here’s the service manual for your boiler [http://bostonheatingsupply.com/Burnham/Burnham%20Independence%20I&O%20Manual.pdf|Burnham Independence Boiler]
Pressure valves are a very tricky thing. If not installed properly you can cause big issues!
I strongly recommend you get a trained heating technician in to install it for now. If you really want to learn go to a good vocational school to study heating.
Just the other day a home near here blew up! The homeowner tried fixing his boiler and now he’s homeless!