To yodel, sing a scale continuously upwards, until your voice "breaks" (switches octaves) into your "head voice" (also known as falsetto in men). This point is your "voice break". Then go back down a note, and up again, over the voice break. Do this loudly, many times.
In "Hodl - Ay - EE - Dee", the "EE" switches to the head voice.
Yodeling has developed in two main styles: the country/western style and the traditional Swiss/alpine style. Some good examples of alpine yodeling can be heard in the songs of Franzl Lang, Stefanie Hertel, Zillertaler Schürzenjäger, Ursprung Buam and Mary Schneider. Examples of country/western yodeling can be heard by Kerry Christensen, Elton Britt, Wilf Carter, Slim Whitman and Patsy Montana. Imitating these singers is the best way to expand the range and complexity of your yodeling.
"Appenzeller" and "Bravourjodler" are yodeling standards which are performed by many different singers.
Good places to yodel are anywhere with an echo. A lake, a rocky gorge, anywhere with a distant rock face, the outdoor areas between office buildings, in a canoe next to a rocky shoreline, or down a long hallway. Amongst the best places to yodel is on a mountain.