Let me suggest a possible solution to avoid drying paint on the cardboard. It is totally opposite from what Bob teaches. But it should work well - I use it when i need to block in dark colors.
Sky: Apply liquid white only in the sky area and do what bob instructs to produce nice sky.
Ground.: No liquid white and no liquid clear. Use older bristle brush and dark purple/blue/brown in single color or in mixes with each other. Now what i suggest to do: Add a bit of OPT (odorless paint thinner) to your paint so it is like a hair conditioner in viscosity or drinking yogurt. Use your old bristle brush and scrub it into the ground level very thinly. There will be brush strokes visible - it is alright. Main condition is to do a thin layer - any dark color. now let it dry for 30 min and take that time to think what will be where.
What is going to happen - because diluted paint application is easy and you scrub it in it gives a thin paint layer. OPT will evaporate and leave you a controllable surface to which thicker paint will stick well and will not be badly mixing with upper layers.
Very often beginner painters do not have enough darks in the shadows due to loosing all the darks when mixing in highlights.
This should protect you from that problem.
Overall - Daler Rowney Artistics are just a bit more oily than Winsor and Newton Artisitc paints I have. But in the current technique i am exploring I am intentionally thinning paints down with different mediums. So if you do not plan return you may keep your paints for future artistic grows that surely will follow when you will start exploring different subjects to paint.