Like any animal sometimes they will need a wash. Some might never need a bath whilst others may be more in need! Remember though that they will always have some smell and there is no need to clean them every day. They wash themselves anyway (like a cat!) and you don't want to be washing out natural oils.

If you think its time ratty had a wash then you will need to do it on a warm day or in a warm room so they don't get cold. Some rats don't like water and if this is the case don't force them because this will stress them out.

Use warm water and supervise the whole time. If you want you can also use some rat shampoo as this will make them smell nice nad their coat look shiny.

Why not make the bath into a game? They'll enjoy it more.