Find your voice break. Sing the note "ay" from inside your chest rising up to your head until your voice breaks. The voice you speak from your chest is your chest voice. Likewise, your head voice originates in the back of the throat rather than the chest.
Sing the notes "ay" in your chest voice and "ee" in your head voice. Push the note "ee" just above your voice break.
Shift the notes back and forth in succession as you sing. Do this for a while to practice. You may also sing the phrase "little old lady," pushing the last two syllables into the head voice.
Add the note "te" to your yodel. Experiment with different chest-voice successions of the notes "ay," "ee," and "te."
Speed up or slow down your variations of the notes in accordance with preference. Cowboy yodels can be fast or slow.