The Shulchan Aruch and most poskim hold that when you pour a reviis (at least) of water onto both hands together or a reviis on each hand separately, your hands become tahor and the droplets remaining on your hands are also tahor. This is highly recommended. If you pour less than a reviis, even if your whole hand gets wet, only the hand is tahor but the residual droplets are tomay. You must be cautious of the following: (MB 1,16)
Do not let those drops run first beyond the point of washing and then return onto your hands. They will re-metamay your hands. If someone can pour for you, keep your hands pointed upwards from the washing until the drying. If you have to pour for yourself, keep your hands pointed down until after the second pouring which will metahair the tomay droplets. (ShA 1 – MB 3,7)
You must pour a second time to be metahair the tomay droplets. This rinse must also cover all of the skin (at least up to the palms – see above #87) where the droplets are. (ShA 2 – MB 19,20)
Do not touch your hands to one another while there are tomay droplets on them. The droplets will re-metamay the other hand. You will need to dry your hands and begin again since no amount of water will be enough to be metahair the tomay droplets. (The second pouring has the ability to be metahair droplets that absorbed tuma from the hand they are on, only. If the droplets absorbed tuma from a hand other than the one they are on, they cannot become tahor.) (ShA4 – MB36)
If you wipe those tomay droplets (before the second pouring) on another surface, do not touch those droplets! They have the ability to make (anyone’s) hands tomay. If those droplets get onto your hands, you must first dry them and then start the netilla over again. (ShA 8 – MB 61)
You can’t wash with less than a reviis if you have something on your hands. If you have an object on your hands that is not a chatzitza (e.g. ring, bandage), the tomay droplets from the first pouring that are on that object cannot become tahor with the second pouring. The droplets on the object will come on to your hands and re-metamay them. (ShA 9,10 – MB 65)
Do not touch the cup handle with your wet right hand before the second pouring and then touch the handle with the left hand (in order to wash the right hand for the second time – i.e. alternate washing). The left hand will have picked up tomay droplets from the right. Dry and begin again. (MB 49)
If your hands happen to be wet (e.g. washing for hygiene) before netillas yodayim you don’t need to dry them first. (BeH “hanoitel”)
Make certain that the water comes upon your fingers and all sides of your hand. Not like many people do – that they pour only on the sides of their hands and are not careful that the water comes upon their fingers and all sides of their hands. (MB 9)
L’chatchila, pour a second time on each hand even if the first pouring was a reviis. This is to fulfil the opinion of the Raavad and others. Since most poskim are lenient when you have a reviis, only do the second pouring if enough water is available. You don’t have to bother looking for water. (MB 21)
Make this chumra on condition not to be bound by a vow. (ShaH 18)
It is best to pour from a vessel that can provide a wide pour to splash the water on your entire hand all at once. To spurt water repeatedly until your entire hand is wet is valid. But if you poured only on a portion of your hand, then you waited until it was dry until you poured over the other portion, the netilla is invalid. (ShA 3 – MB 30)
It is a mitzva, l’chatchila, to rub your hands together after you wash – for extra cleanliness. (Rama 2 – MB 24)
If your hands are still wet and someone else’s tomay hands touched those droplets, your hands become tomay again through those droplets. Dry your hands and start all over. (ShA 4)
After washing one hand it can become tomay again if the other hand touched it while still tomay. (ShA 6)
You do not need to dry the handle of the vessel before you begin washing. (See above #90 d,f for instances where you would need to dry the handle.) The tuma on your hands will not transfer onto the water that is not for netilla. (ShaH 41)