How to come up with keywords?

I need help. I don't understand how to choose long tail keywords to optimize for. Can someone give me a bullet point explanation of how to do it? Here's what I do so far: Let's take a book summary app \- Start with the keyword book summary app \- Check google keyword planner for any long tail keywords with low competition and high traffic \- Plug that into google trends \- Choose the keywords from that list with high interest How do you guys choose the right keywords?