Warm up your voice. Do some scales across your full range. Start very low and go as high as possible for at least 10 minutes. Sing from the diaphragm instead of your throat to prevent harming your vocal chords.
Sing a very low note, deep from your chest; hold it, and then break into a falsetto. Go back to the lower note, hold it for several seconds, and then go back to the falsetto. Choose a very high note if you can't find your falsetto. Make an audible distinction between a guttural sound from the chest and a high note from the head.
Crack your voice between low and high. Going from low to high is just singing, it isn't yodeling, so crack your voice as you go into the falsetto. There should be a prominent, audible break between low and high. Tense your diaphragm muscle and practice moving your throat during the break to achieve the crack.
Watch online instructional videos or take lessons to hear the three main notes in yodeling. Once you know the basics, go up and down in varying rhythms to get the perfect sound. Practice different varieties of yodeling, such as Alpine, American Cowboy and comparable styles from Persia and Zimbabwe.
Get some music. Buy an instructional yodeling CD and sing along with the music. Visit a German restaurant or brauhaus in your town that has live music and ask if they have yodeling performances.