There are two ways to use a stethoscope -- the first is for medical uses by a medical professional and the second way is for a normal person to listen to their bodily sounds.
Since I hope a medical professional is not finding out how to use their stethoscope by reading about it on the net, I'll assume that this is for a somewhat normal person.
It is easy to use -- simply make sure that there is a protector on the ear pieces and put them in your ear. You can then hear a variety of sounds simply by putting the round piece on something and listen. This opens you up to a whole new world.
Both of my children are in the medical field and they received their first stethoscopes as Christmas presents. They became a party game and we listened to everything we could. That is the best way -- just put it on something and listen.
If you want to listen to your heart just put it where you think your heart is and if you don't hear it move it around until you do. This is a case where experimenting is fun.
Although listening to the heart is amazing and makes you feel a part of the harmony of the universe, it is more fun to listen to your stomach. If you are having trouble locating it, take a drink of beer and listen for the sound of that entering you stomach. The stomach will make several different noises depending on how hungry or full you are.
The last obvious place to listen is your lungs and breathing. This one is eaiser to do on a willing subject since you tend not to breath as deep as possible while you are trying to locate the best place to hear.
Once you are familiar with the sounds of the human body, hunt down Fido (preferably when he is napping) and listen to the differences of the canine dog heart beat and lung sounds. We never tried it on a bird, but I bet that you could.
It isn't hard to use a stethoscope -- just put the earpieces in and start listening!