1- Shut off water supply
2-drain water possibly one floor below the fixture your working on
3-have a basin wrench close by
4-disconnect the water supply from the faucet (you may have to lie on your back as you use the basin wrench.
5- Using TWO wrenches disconnect the supply piping and then unscrew the valves holding back as to not break the threads or possibly screwing the piping out of the wall
6- Using Teflon tape or other thread sealant wrap the exposed threads and screw on the new valves again holding back on the nipple protruding out of the wall.
7- close the new valves turn on the water supply and let out the air on the new valves with a bucket up it and check for leaks.
8- install a new supply copper tubing Chrome plated or CP brass nipples DO NOT use the crap stainless steel or plastic flex junk sold in big box stores.