Yodeling (or yodelling, jodeling) is a form of singing that involves singing an extended note which rapidly and repeatedly changes in pitch from the vocal or chest register (or "chest voice") to the falsetto/head register; making a high-low-high-low sound. This vocal technique is used in many cultures throughout the world.
Easy Steps to Yodel
Below are simple steps to learning how to yodel. Go step by step, and check your progress.

1. If you want to yodel you will need to use a special part of your voice. This is the same part of your voice that you use for yelling brief words at your friend or neighbor. Words like “GO,” “HEY,” “NO” and other short words usually depend upon the top of your lungs.

2. This is another good way to locate that special part of your vocal cords that is used to yodel like the pros is to loudly hum. You have to find a word or sound that changes pitch at least twice and hum it loudly. This basically a yodel without words!

3. So, now you’re ready to yodel for real and go outside (somewhere secluded) or if you’re really brave you can try in the shower. Give your yodel a try from the top of your lungs. Try to alternate pitch and give it some good resonance. Practice for a good long period of ten to fifteen minutes.

4. The last step is to practicee and everyone has a special yodel that will sound different, but with time and practice you can hone your yodel to sound like the pros. After only a few days of yodeling it should already feel more natural on your vocal cords. Practice on a few different occasions. After you have practice two or three times you should notice a lot of improvement.
Source: learnthis.org