An overview of 'how to make a painting' in 5 steps...
1 Find the 'main idea'
2 Design (gather reference, sort material, adjust for good design lines and shapes)
3 Light source
4 Plan value
5 Pre-mix Colors
Gather material that is important to you. This can be many sources and might make several paintings. To decide how big an artwork should be is not as important... maybe try several sizes!
It's best if you have been to the location and have a personal memory of why a scene is attracting you. Work through some lists, sketches or doodles until you find your main idea.