Project your voice. You need to use power in your voice to yodel. Build from your chest, not your vocal chords, to achieve the depth and volume you'll need to yodel.
Practice your falsetto. Yodeling requires a broad range, going from high notes to low notes in relatively quick succession. Vienna Online even suggests trying to imitate the noises of a chimpanzee at the zoo to practice the notes you need to hit for yodeling.
Switch back and forth between the deep, powerful yells from Step 1 and the falsetto notes from Step 2. You may want to practice some traditional yodeling sounds to get the hang of it, such as "yodel-ay-ee-oo."
Continue practicing, and take deep breaths while yodeling. It may take time to develop the necessary stamina to yodel.
Listen to country-western yodeling music to yodel as cowboys do, after you've mastered the basic yodel. Try to imitate the sounds you hear from cowboy yodeling. Add a country-western twang and style to your yodels, based on what you hear in authentic cowboy yodeling.