The first rule is about who is supposed to throw the baby shower. Traditionally, the shower is supposed to be hosted by someone who isn't in the direct family of the mother-to-be. This could be a friend, co-worker, or family friend. Nowadays, it is typically okay for anyone except the mother-to-be to throw the shower. The most common person to host the shower is either the best friend of the mom-to-be.
The next rule is about the timing of the shower. Typically, you will want to throw the shower within the last few months of the pregnancy. You want to be sure not to have the baby shower too close to the due date just in case the baby decides to make its entrance early. You also don't want to have the baby shower to early as that is often considered to be a bad omen.
Another piece of etiquette that is important to follow is about who is expected to pay for the shower. Generally, the person who is hosting the shower is the one that covers the cost of the shower. Because the shower is meant to be the gift to the mother, it is often considered to be rude to ask her to pay for it. Additionally, it is considered to be a very tacky thing to ask your guests to pitch in. If the cost of the shower is too much for one person, it is often a better course of action to ask a few others to help you co-host the baby shower.