Learn French: Détends-toi avec un massage doux.

This is the rule for affirmative commands (telling someone to do something). The reflexive pronoun moves to the end of the verb, attaches with a hyphen, and te changes to the stressed form toi. If it were a negative command (telling someone not to do something), it would stay in its normal spot: Ne te détends pas.

I learned that the tu form drops the 's' in commands. Why does détends still have one?

The rule about dropping the final 's' only applies to -er verbs (like parle or lève-toi). The verb here is se détendre, which is an -re verb. For -re and -ir verbs, you keep the 's' in the tu command form.

Why is the French verb reflexive (se détendre) when the English translation is just 'relax'?

In French, many verbs describing a change in your own physical or mental state are reflexive, literally meaning 'to relax oneself'. If you just used détendre without the pronoun, it would mean to loosen or slacken a physical object, like a rope.

Does doux end in an 'x' because it is plural?