Who it's for, and when
Who it's for: everyone, with a partner or solo, beginners included. To pleasure themselves, most women simply use their fingers. The clitoris isn't the only instrument of pleasure, but it's at its heart, and often what sets it off: it's the only organ in the human body with no function other than pleasure.
When: as foreplay, after long caresses all over the body, never straight away. On your own, it's also a chance to find out what you like, so you know what to ask your partner for afterwards. Above all, it takes patience and attentiveness.
Precautions: the sensitivity of the clitoris varies enormously from one person to the next. For some women, direct contact hurts, irritates or just feels too intense, while for others it sets off a fit of giggles. Desire has its eclipses too: tiredness, stress or depression can snuff it out, and the pill dials it down in some women.
Persistent pain when touched is worth raising with a doctor or a sex therapist, as are questions about pregnancy or limited mobility. For hands: clean, with short nails. For lube: a water-based gel, never petroleum jelly or baby oil.