You need a second person. Cat clipping requires 4 hands to stretch cat out, hold in place and clip without cutting them. Son helped me with our furry old girl so I could get the mats off her stomach. I comb her but she objects to stomach combing, hair pulling and two hands to hold her in place are not enough! She didn’t mind being held upright in place, stomach exposed, while we got comb under her various mats, then clipped them off. She does not like being put on her back with stomach exposed, but standing up on hind legs is fine.
Precautionary note is to clip claws short beforehand. She had gotten a bath two days previous, so the stubby ends were not lethal! But some cats are terrible when they have long claws and object!! She gets claws clipped regularly, a bath about once a month, no issues with that. She is REALLY hairy, and with age, 14-15yrs, she is not as good at self-grooming as she used to be. Cut off claws help prevent furniture damage too.