How do i install a Pellet gun ram air spring?

Short answer - you probably cant. In most cases it requires a special press to hold the main piston in place while the spring is compressed, then relaxed do it can be removed. People who try it themselves usually end up breaking their expensive air gun and hurting themselves in the process. If you are an accomplished mechanic and have a fully equipped workshop, consult the manufacturer for instructions, but they typically will not give them out due to liability issues - its just that dangerous.