Grounding a potmeter is often done by soldering the groundwire to the back of a potmeter. This is a fine method but you will to have the right solder skills.
An alternative grounding method is by using a solder lug washer. Replace the plat washer that comes with your potmeter with a solder lug washer and solder the groundwire to the lug. The pot is now grounded. This method is fast, easy and clean.