Whether you are repacking a tap, radiator valve or a gate valve the procedure is the same. However you cannot use regular p.t.f.e tape on potable water.
Turn off the water supply to the valve your working on.
Drain off water to the valve being repacked.
Turn the valve off (by turning clockwise) so the spindle goes into the valve.
Undo the top/cover and remove so access is easier.
Undo and remove gland securing nut (the nut holding the spindle).
Remove old packing/washer that was under the nut at base of spindle.
Replace with new washer or a suitable material like p.t.f.e. tape.
Put valve back together.
Fill and check for leaks.