It is easy as pie. At a minimum you only need to have an RO community string. The community string is like a password that is used to validate SNMP requests. RO means you are only allowed to read the MIB variables. RW means you can set and read MIB values. In SNMP set=write and get=read. The following config sets up both a set and get "password" It also. By default most applications use public as the RO and private as the RW. You
snmp-server community myROpassword RO
snmp-server community myRWpassword RW
You can set up traps and other things but it should not be necessary for the PRTG application. I believe PRTG only allows gets, so you will only need to use the RO line, even though the RW line works as well.