probably crap tips but I found these helpful for decorating decisions which can get overwhelming.
I used my work ppt skills to build mood boards of ideas using images of stuff from the internet to work out if my choices would go together - I found it easier to see and then helped me quickly rule things out if I was getting overwhelmed.
The 70/ 20/10 idea is good when thinking about colour schemes and proportions. I found it worked as a guideline. So I tended to stick to three main colours for walls and decor. In vaguely those proportions.
For example:
Hallway 70% yellow ( walls)
20% grey ( striped carpet / small furniture)
10% dark grey ( woodwork)
Loft bedroom:
70% grayish white ( existing furniture and 3 walls)
20% bold blue ( feature wall)
10% chalky pink ( curtains)