"bigwelcome: to the Paint Basket Vicki
Good on you for having the courage to start chatting
If you are painting in one sitting then you could just add water as needed to thin the paint to whatever consistency you need at that point because all the paint will dry at the same rate as is has been painted at the same time.
If however you are painting in multiple sessions, then subsequent layers need more oil than the previous to prevent cracking.
What you then do is add some oil to your water before you start painting. That way everything you paint on this this layer will have more oil in it than the previous layer.
If you need to stop and paint in another session, so a day or more later then the next layer needs more oil (fat) than the previous two so just more oil to the water than the previous layer and so on.