Learn Italian: Sciacquare il viso e lavare le mani.

This is the jussive infinitive. In Italian, when giving general instructions, rules, or warnings to a general public (like on a sign, a recipe, or a manual), you use the infinitive instead of conjugating the verb into the imperative.

Why does the sentence use il viso (the face) and le mani (the hands) instead of possessives like your face and your hands?

In Italian, when referring to body parts, you almost always use the definite article (il, la, i, le) instead of a possessive adjective (il tuo, le tue) as long as it is obvious whose body part is involved.

Why is the feminine plural article le used with mani?

Mano (hand) is a highly irregular noun. It ends in -o in the singular but is feminine (la mano). In the plural, the ending changes to -i, but it keeps its feminine gender, making it le mani.

What is the exact difference between sciacquare and lavare?

Lavare means to wash, which usually implies using soap or a cleaning agent. Sciacquare means to rinse, which specifically means using water to clear away soap or dirt.