I personally never use an alarm for anything so perhaps someone else can comment on that.

The most important factor is the desire to know when you are waking up the moment it happens, and to set that intention each time you fall asleep after a forgotten or failed DEILD (or after your alarm wakes you up).

The dream you enter can be anything. If I notice that a lucid dream is fading, I often just wait until it forms again. If it is not a lucid dream, then I often exit my "body" from the bed as soon as I become aware, or you can just WILD into any visualized dream; whether it is the dream you just left or not. I like to create landscapes I want to visit during the day in my mind and focus on them for several minutes so that I have them ready when the time comes.