grulla,
Not foolproof or sure-fired.
There are many different techniques...it depends on what size/age the animal is, and what works best for you regarding your facilities and abilities. Strength is not much of a factor, as long as you are not riding and roping the critter.
The main thing that I believe is important, is that you have a tube that is made for tubing, its beveled on the end, for a smoother entry. Lubrication is very important, use some KY or other water based lube, not oil. Oil can be deadly in the lungs and the tube will go into the lungs sooner or later.
Grease up the tube, and remember...have a destination. Maybe you want to fix bloat...go in several feet....or, to re-hydrate, go just past the mid-throat. Rub the cows neck as you push it in, and look for swallowing, its a good sign. Choking and coughing means pull out, you are in the airway. Usually the tube will go down the right way.
You will have to get the cow in a headgate...and tie up her head. A calf is much easier than a full size bull...obviously. They do not enjoy the procedure, nor will you, but it is quickly forgotten by most cows, and they realize you helped, not hurt them.
Air sounds and moisture (hold a mirror to the end of the hose, it will fog up a tad most days) and you are in the lungs.(Don't worry about hitting them).
Stinky smell and sounds that don't correspond directly to breathing...you are in the right spot.
Others will add their words, most more experienced than I.
One question...Why are you tubing this cow?