Understand that you have two voices available to you in singing. The first is your chest voice and the second is your head voice. The chest voice is where you comfortable sing at and if you're aware, you can feel the reverberation in your chest. Your head voice starts as you go up the scale and come to the top of your chest voice. It feels like singing through your nasal cavities and has a different sound.
Sing "yo-dah-lay-hee-hoo" all at one note in your chest voice. Middle C is good for the ladies and tenor men. Low C is good for bass men. Notice how your body feels, especially your chest area.
Change a couple of the notes to a much higher pitch. For example, sing "dah" and "hee" at least an octave up. It doesn't really matter what notes you sing; there's no music to read. Sing low-low-low-high-high (slightly lower). This is the basic yodeling format.
Vary your wording, tempo and your notes as you progress. You can have several notes the same, jump up to a high note, back down to a low note, etc. As long as you keep jumping back and forth, you're yodeling. You can use just about any "hoo," "ha," "lo," "dee," "ee" type sounds, but not actual words.