Value and color mixing is key to understanding how to work with the challenges unique to gouache. The value shift (2 values) as it dries can be very frustrating, but knowing color mixes and using a neutral gray can help.
Try to not have a set notion of how your painting is going to turn out. Explore and layer. Be ruthless about re-doing areas and merging. Heavy manipulation can be good for gouache. Reserve judgment about your study until it's had time to fully dry... sometimes this takes walking away for an hour and coming back! It usually looks better than you think it does when allowed to dry fully. Watch out for adding too much water. Use less than you think and less as you go on in subsequent layers.