I use a third hand clamp which is spring loaded to hold the spring at one point giving you both hands back,if you google ROU38332 you can see the type i’m useing
Small F clamps such as Axminster ones are great. Sometimes I even use two. They can tightened right up so that the pressure of the spring being fitted doesn’t pop the clamp off. Also on fagor and samsung machines, they’re great for supporting the bottom of the ring while its fed into the right position before tightening.