Sing "ay" and hold it for a few seconds. Notice the low sound of this note and that it is originating from your chest region. Focus on your chest when making this note.
Sing "ee" and hold it for a few seconds. Notice the higher sound of this note and that it is originating in a higher region of your vocal chords. Focus on your head when making this note.
Sing "ay" for two seconds and then switch to "ee" for two seconds. Focus on the point at which you change to the "ee" note. Concentrate on making that change as quickly and seamlessly as possible. You won't be able to do this very well on your first few tries, but keep practicing and you'll get it.
Sing "ay-ee" quickly together and repeat the sequence at least five times once your transitions have become rapid. Continue to focus on these transitions.
Sing a longer stream of notes once your transition from chest to head region is quick and seamless. Try singing "yo-dah-lay-hee-hoo." Continue to focus on where these notes are being produced and where the transition happens in the head region.
Vary the tempo and notes you utter as you progress. Try combinations of "hoo," "ha," "ee," "dee," "lo," "ay" and "da" sounds in differing order.
Practice your yodeling at least once a day for at least 10 minutes. The only way that you will progress as a yodeler is by training yourself and your vocal chords to make the notes you want.