If you do something comme il faut, you do it in an acceptable way, following the rules or traditions. It's comme il faut in ballet to wear your long hair in a tidy bun.
An etiquette expert can tell you what's comme il faut in various situations — in other words, the correct or polite way to do things. It's comme il faut at a swimming pool for boys to leave their clothes in a pile, except for their swim trunks, and jump into the water. At a wedding reception, that wouldn't be comme il faut. This phrase, used in English since the 18th century, means "as it should be" or "as is necessary" in French.