These are a few flower paintings I did last year. I noticed that the greens could be cool or warm and the colors of the petals had temperature changes.
Work from life as much as possible. Use your cropping viewfinder and edit out anything extra. Make a thumbnail study of the values. Pre-mix piles of the colors you'll need using a palette knife. Do lots of small studies.
Here's a beautiful arrangement of flowers bought at Trader Joe's.
Practice drawing parts of it. Another tip is to use your cell phone to photograph and crop your composition.
See what it looks like in black and white.
There's an awesome phone app called: Notanizer. Costs $2.99 and will put your photos into a notan, 4 levels of black and white and allows you to see the simple shadow shapes of your composition.