The primary decision to be taken by the investor is the amount he is going to invest every time. The amount should be chosen such that it will be affordable regularly over the long-term basis.
Then the investor should decide how frequently he is going to invest such as each month, each quarter, each six months.
The basic principle is the investment should be done without fail irrespective of market fall or rise.
The day-to-day fluctuations don't matter in this concept. Thus investor should have long-term perspective.