The most important thing is to translate idea (typically, a solution) into an underlying problem. Then, validate the problem:

    Who has this problem?
    Talk to them, do they really have this problem?
    How many people have this problem? Would they pay for a solution?
    Is there another solution for the same problem
    that is easier than your initial solution?

After that I just make a pause. After a few days/weeks I revisit the idea with a fresh look.