Custom themes: Where to start

Hi guys,

I’ve been setting up some WordPress websites for customers and have used different themes each time. I used a lot of time in these themes code to customize things and therefore I want to make my own themes.

My question is, how can I strip down a theme, ex.Twenty Fifteen, to a minimum and then customize it for my own needs? What would be a good starting point when to make custom themes?

Thanks for any input. 🙂