Do you have any idea on how to install a stop valve under a bathroom sink?

Thanks R. Koehn

Do you have copper that needs to be installed using a compression fitting, or galvanized pipe that is threaded? Turn the water off at the main, remove the fitting going to the faucet and install the valve, then reconnect the faucet. Take a picture and go your local hardware store, they can help you get what you need.
Good Luck!

If You Have Cpvc Pipes , Purchase A Male Adapter, And Glue It Onto The End Of The Pipe, Then Insert The Riser Tube Through The Nut And Ferrell And Tighten. Then Turn The Water Back On.

What kind of pipe are you putting the new valve on, copper plastic or iron pipe?
Let us know.