its easy.. get in a comfortable position... relax... breath deeply and focus on whatever you want or dont want...
like replay all you did that day...
or count your breaths and each time imagine your falling deeper into the nothingness.
or focus on the whirling colors behind your eyelids..
or on you appendages feel every hair, slight air current and imagine that your spirit is moving independent of you body.. rise from you body.. etc etc..