When both a direct and indirect object pronoun appear in a sentence, the indirect object pronoun will precede the direct object pronoun: Me lo dice; Te la mando ahora. When both the direct and indirect object pronouns are in the third person, the indirect object pronoun must be replaced by se:

le/les & lo/la/los/las–se lo/se la/se los/se las

¿Le das los libros a Silvia?
Are you giving the books to Silvia?

Sí, se los doy.
Yes, I’m giving them to her.

¿Les compras los juguetes?
Are you buying the toys for them?

Sí, se los compro.
Yes, I’m buying them for them.

¿Me trae un café?
Will you bring me a coffee?

Sí, se lo traigo ahora.
Yes, I’ll bring it to you [formal] now.

Note that when object pronouns are attached to affirmative commands, present participle, or infinitives, you may need to add an accent mark to preserve the stress of the word: Búscalo; Estoy buscándolo; Tráigamelo; Voy a traérselo; but Voy a traerlo.