The undertank filter is K&N # RU-5111
Basically you remove the snorkel tube and plastic housing that connects the stock air filter (underseat) cavity with the undertank tubing that goes to the airbox on the side. Then fix the RU5111 undertank and there you go.
I know, my descriptions are a tad cryptic. Pics are worth more in this example. I don't have any but there are threads around that detail this mod.