What Mike said is the way to do it. I did mine about 6 months ago and the five tires are not leaking. The key thing is make a small hole, like Mike said. The hard part is getting the the metal core out of the tube. I ended up using a small screwdriver and pushed it in around the metal stem and finally broke them loose. If you are like me, you can't believe you could get the big inner part of the metal valve stem through a tiny hole. I clamped the metal valve stem upside down in a vice (with pieces of wood to protect the stem), soaped it up, like Mike said and the tube stretches over the little hole easily. The last thing I was concerned about was how tight to tighten the nut. I just tightened it very firm but not so I flattened out the rubber a lot. Hope this helps.
Answer to Mikeinnj. The rubber valve stems have inner metal cores.
Rusty Nelson