Don't do too much for now. I've got valve stems and a rim or two so I'll measure it. Normally you'll just lube it a little with anything suitable for rubber, silicone grease, tyre lube, lanolin grease, soap or whatever. The valve stem is inserted from the inside and pulled through to the outside using a suitable puller made for the job (it screws on where the valve cap fits). Fitting it takes perhaps five seconds all up.