How is the fasting of "alternate days," which is the dearest fasting to Allah, achieved: Is it by observing it consistently throughout life, or by performing this act of worship for a specific period, and if so, what is the duration of this period (a week or a month)? And what is the best day to start this fasting?

Fasting of Dawud (peace be upon him) is the best fasting in the sight of Allah. It consists of fasting one day and breaking fast the next. It is harder on the soul than perpetual fasting because it does not become a habit. There is no specific duration for Dawud's fasting, nor a particular day to start it. Whoever fasts one day and breaks fast the next has done what Allah loves and will be rewarded for it. Allah rewards good deeds, even if they are few. It is not a condition for Dawud's fasting to continue for a lifetime to earn its reward.