To make kefir you will need a starter and milk. 

Kefir starter can be purchased in dried packets or made from a starter or from kefir grains.  Kefir grains look like little gelatinous flowers (like a cauliflower). 

Unlike yogurt, kefir does not need to be incubated.  It will ferment at room temperature.

To make kefir, mix a starter packet in milk and leave at room temperature for 24 hours.  If using a starter culture, mix equal parts of milk and culture and let sit at room temperature for 24 hours.  If using grains,  add milk to grains and let sit for 24 hours at room temperature.  Strain off the kefir and add  more milk to grains.  Keep grains in milk in the refrigerator.  You will periodically need to add fresh milk to the grains to keep them alive. In all cases, shake your kefir several times during the 24 hour fermenting period.

If you use a starter culture, make sure you keep some on reserve to start your next batch of kefir.  Refrigerate after kefir is fermented.