O-Ring Mod on my CRF250X

Can anyone tell me how to perform the o-ring mod on a crf250x. I tried to do a search on it but was not successful.

The bike has already been jetted but if I do the o-ring mod do I have to re-jet again?

Also can I use any type of O-ring or does it have to be specific. Meaning does it have to come from a specifc company or can I get the sizes needed at the local hardware store?

Any help would be great.

Thanks