My Twinn dolls are all held together with an internal skeleton or armature structure. The dolls aren’t made any longer and so you can’t replace a lot of things, but if you are wondering about how the feet work it’s almost like a plug and a socket.

The plastic knob fits into the cup shaped part inside the foot. If you are replacing a foot that maybe got chewed or colored on with a new foot you will need some muscle to pull the old foot off. It will pop off and you can switch the foot out. If the foot is already off you can use the next steps to put it back on.

It is not possible to just push the foot back on. Even if you try to do it super strong, it just won't click back in. Mostly it is because the leg connection is too far inside the leg to reach all the way into the foot cup. You will need a blow dryer on hot setting and blow it at the leg connection for a good 10 minutes until it is soft. This will actually heat the temperature of the entire leg piece, so don't be alarmed if the vinyl shifts up or down. Once the leg is soft, push it upwards so that you expose the full joint of the leg connection as seen below.

If you need a good grip on it, you can push it up further to reveal the skeleton armature. Take the foot (be sure it's facing the right direction) and hold the connection firmly and push the foot up onto it. You should hear a snap when you've done it correctly. Then move the leg vinyl back down to its original position. As it cools the original glue that held the vinyl in the correct place on the leg will re-bond the leg vinyl so that it's not moving. If you need to add extra glue you can, but there should be enough previously to keep your leg completely in tact.