If you have any waterstaining then you need to treat it before decorating or it will just bleed through the paint. This will happen within minutes. The best way to do this is to use an oil based paint of any description. However that will always need to dry overnight.

So if you need something faster than that, use Zinsser BIN. It’s available in any decorator’s shop and it can be bought in pretty small amount. It’s probably easiest to use a disposable brush to apply it and it dries within 30 minutes. It’s probably safest to allow 2 coats. Wrap your brush up in plastic to keep it ready since it will harden within minutes.