Learn Italian: Non lasciargliele! Tutti usano le graffette. - Do not leave them for him! Everyone uses the paperclips.

For the informal tu form, a negative command is always made using non + the infinitive of the verb. So the negative command for leave is non lasciare.

How do the pronouns combine to make gliele?

It combines the indirect object pronoun gli (to/for him) and the direct object pronoun le (them, referring to the feminine plural le graffette). When gli combines with a direct object pronoun starting with l, it changes to glie- and merges into a single word: gliele.

Where do we place these combined pronouns in a negative tu command?

You have two choices! You can attach them to the end of the infinitive by dropping its final e (non lasciargliele), or you can put them before the verb (non gliele lasciare). Both are perfectly correct and common.

Why is the verb usano in the plural form when the English translation says everyone uses (which is singular)?

In Italian, tutti literally translates to all or all people, which is a plural concept. Because of this, it always takes a plural verb, unlike the singular subject everyone in English.