I believe they are friction fit. Just slide them on.
Most people use them to protect the ends of their barrels (as some Palmer marker's are fixed barrel).
Others appreciate the added length in speedball (bunker pushing).
RamboPreacher reportedly used a dB meter and found no difference in sound when using them.
If you are getting porting, I would recommend against it. You would defeat the purpose of porting yoru barrel (as friction fit will cover the porting holes).