How many layers of glue do you add? also someone mentions care during install of tires (stretching?) because they could be damaged. Can I damage the tire by stretching it too much?
Because when I installed new Continental Sprinter 250 it goes over the rim very easily. Is this normal? It was easier than lo end Vittoria I installed long time ago. It was very elastic, like a rubber band.

I prefer 2 thin coats on both the rim and tire and then one additional thin coat on the rim prior to mounting. It's possible to damage a tire by stretching too much but I suppose it depends on how you stretched it. If you mounted the tire on a rim and let it sit for a month, no worries. I would say that if the base tape has not pulled up or split, you're probably fine.

For some good, easy to read directions IMHO go to the Zipp website and download the owners manual for a set of their wheels. I think they cover it pretty good.

The number of coats sounds about like what tone12 posted IIRC.