During my Jimmie Rodgers period I had to learn to yodel and I chose as my teacher Bill Staines.( Lofty goals) When thast failed I tried this. Sing the vowel "A" in a comfortable point in your rage then slide the "A" to "E". The natural tendencie is for the voice to lift slightly. As it lifts force it to a falsetto. When you can make this break cleanly move to the vowel "O" and with the same process slide from "O" to "OOOO". A little work to clean up the slides and a little imagination you should be able to figure it from there.
Cathy Fink has an excellent audio tape on yodeling. I reccomend it highly.