How to put an RC Circuit (high p filter) in my Electric Guitar?

This doesn't sound right. For a high p circuit you need a cap in series with the circuit. When you take a cap to ground you have a low p circuit. The high frequencies would go to ground through the cap. The wikipedia link below shows a high p circuit just to make sure. It also gives the formula for computing the values you need to make your own filter at the frequencies you want. If you need voltage to still get through the circuit you need to byp the cap. Otherwise the cap will block any DC.