One must immerse the entire utensil in water in one shot. If a part of the utensil is sticking out of the water, the tevila is not valid. If part of the utensil was sticking out then the impurity which rests on that part spreads back through the utensil when you pull it out of the water, and the part that was already immersed would require a new tevila.
The utensil must be clean, with nothing on the utensil which would prevent water from touching even a small part of it. All stickers and labels should be removed prior to tevila, as should any dirt. If one washed off all stickers or dirt and there is a stain left behind, it is not a problem. If one cannot remove all the residue, a rabbi should be consulted.
When immersing the utensil, it should be held loosely. Holding the utensil too tightly causes one’s hand to prevent the water from touching the utensil where it is being gripped, and the tevila is invalid.
It is a good practice to wet one’s hands before the tevila, so that if one mistakenly grips the utensil tightly, one’s hands would not be considered a barrier between the water and the utensil being immersed. By having wet hands, the water inside one’s hands connects with the water of the mikvah and the tevila is valid. One must be careful to wet their hands in the water of the mikvah, and not remove their hands from the mikvah until after the tevila is finished.
The Taz rules that although one may hold the utensil if his hands are wet, if the utensil is gripped too tightly then the water of the mikvah cannot connect with the water inside his hands. This would make the tevila invalid. Therefore, some poskim write, if one can let the utensil out of their hand for a second during the tevila, it is proper to do so. This way there is no concern that one part of the utensil was being held tight to the level that the Taz would call a chatzitza.
A bottle or similar vessel with a narrow neck should be immersed with the neck facing up, and left immersed in the water until the bottle fills completely with water.
A disposable utensil may be used without tevila since it is not considered a utensil of significance. However, one may not use a regular utensil even one time without tevila, even if one only plans to use it a single time.
If a utensil that needs tevila got mixed with other utensils that had already been immersed, all the utensils require a new tevila.
Tevila does not need to be performed with specific intention to do tevilat keilim. This means that if a utensil fell into water accidentally, the utensil no longer requires tevila. A child may do tevila, but a child cannot be trusted to actually perform the tevila properly. Therefore, a child can only do tevila if an adult was there to witness it. Similarly, a non-Jew may do the tevila if a Jew witnessed that he actually did it.
Bracha on the Tevila
Before doing the tevila one makes the bracha אשר קדשנו במצותיו וציונו על טבילת כלים. The Aruch Hashulchan explains the custom to always say על טבילת כלים – in plural – even when performing tevilat keilim on a single utensil. This is because one is making the bracha on the general commandment of tevilat keilim, and not specifically only on this instance of tevilat keilim.
Sephardim, when only doing tevila on a single utensil say על טבילת כלי – singular.
The Pri Chadash rules that although one should optimally make the correct bracha, either bracha works in any situation. Whether one said כלים when immersing one utensil, or said כלי when immersing many utensils, the bracha is acceptable and does not need to be repeated.
If there is uncertainty whether a utensil requires tevila or not, one cannot make a bracha before the tevila. The Taz writes that a good practice is to combine the tevila on such a utensil along with a utensil that definitely requires tevila, this way one can make a bracha that would cover the utensil which is in doubt as well. If one does not have a utensil that for sure requires tevila at hand, and there is a chance that one may inadvertently use the utensil while waiting to do the tevila, one should not push off the tevila. Rather, one should do the tevila without a bracha.
 חכמ”א מובא בפת”ת שם ס”ק ג
 שו”ע יו”ד סי’ קכ סעי’ יג
 שו”ע שם סעי’ ב
 רמ”א שם, ש”ך שם ס”ק ז
 אג”מ יו”ד ח”ב סי’ פה
 ט”ז יו”ד סי’ קצח ס”ק כז
 עי’ חלקת בנימין סי’ קכ ס”ק כו, חוט שני טבילת כלים עמ’ לו
 שו”ע יו”ד סי’ רב סעי’ ו
 אג”מ יו”ד ח”ג סי’ כג
 רמ”א יו”ד סי’ קכ סעי’ ח
דרכי תשובה שם ס”ק לו. ועי’ באר משה ח”א סי’ מה שאם קשה מאוד לטבול הכלים, נסתלק טעמא דדבר שיש לו מתירין, ובטל הכלי וא”צ טבילה.
 ש”ך יו”ד סי’ קכ ס”ק כח
 שו”ע שם סעי’ יד
 רמ”א שם
 יו”ד סי’ קכ סעי’ כב
 שו”ע יו”ד סי’ קכ סעי’ ג
 שם ס”ק יא מובא בבאר היטב ס”ק ד
 שם ס”ק יט
 דרכי תשובה סי’ קכ ס”ק יט