A piston ring groove cleaner is a simple tool and generally inexpensive. These might be in the $8 to $15 range. The guide side (the "V" end here) will have a couple of thin webs that ride in the ring groove on the piston. The working side (opposite the "V") will hold a small scraper finger that fits closely into the ring groove to scrape out any dirt an/or carbon deposits. This allows you to clean the full depth of the ring groove without damaging the aluminum material of the piston, and so maintaining a clean and proper fit for the gorrove to hold a new piston ring.

This tool should include an assortment of small scraper fingers to match a wide variety of groove widths. Alternately it might have one or two small wheels with multiple scraper fingers around the edge that may be indexed into working position. If you are on a tight budget you can do without this tool. You could use a broken piston ring as a manual groove scraper, but be very careful not to accidentally scrape away bits of aluminum from the sides of the grooves. This tool should be cheap enough to buy one just for the convenience (and on general principals).