The pronouns are appended as suffixes to some grammatical forms:

a) Usually the pronouns are appended to infinitive verbs, it is still considered correct not to append them:

I can see him. Le puedo ver. Puedo verle.   He wants to give it to you. Te lo quiere regalar. Quiere regalártelo.   You can do it. Lo puedes hacer. Puedes hacerlo.   I can write it to you. Te lo puedo escribir. Puedo escribírtelo.   You can tell it to him. Se lo podéis decir. Podéis decírselo.

With a positive imperative (Do it!) it is obligatory to append the pronouns. In an negative imperative they are not appended.

Do it! - Hazlo   Don't do it! No lo hagas! -

In constructions with gerunds, the pronouns are to be appended.
In constructions with estar + gerund it can be appended but it's not obligatory.

I'm doing it right now. Estoy haciéndolo. Lo estoy haciendo.   (While) reading he ate. Leyéndolo comía. -   He is reading right now. Esta leyéndolo. Lo está leyendo.

More details on gerunds you'll find in chapter 8.11.
More details on imperative forms you'll find in chapter 9.3.