How do you come up with ideas?

Don't try to come up with ideas first. They will come, let them, write them down.

But try to see problems.

What annoys you about the world? What is hard? What is easy? Talk to your friends. What annoys them about the world. Then try to come up with solutions to the annoying things.

You should read this article :)
Live in the future, then build what's missing.
http://www.paulgraham.com/startupideas.html

Most of my ideas come from the world I live in.

• I worked as a tester and saw a gap/need for things there to support testers - I ended up doing an online community, conferences, training, online learning platform.
• I once co-started (then quit) a coworking space because it was something I wanted - a local community and a space to work.
• I'd love to start something in the unschooling space as I unschool my kids.
• I think a lot about community building as that is really what I've been focused on the past 10 years, might do something in that space at some point. Maybe, I'm still thinking about it.

I have ADHD and I want to solve the problem which belongs to people like me.

I wouldn't feel stupid about it, here's my usual flow:

1. I come up with some absurd idea of something that I think I need to improve my life and absolutely can't live without.
2. I search the web and find a bunch of tools that I think suck and I could build better. I sign up for most of them.
3. I forget about the idea a few days later, because in reality, #1 was completely false.
4. Occasionally, I just end up putting a bunch of stuff in a spreadsheet and that /usually/ solves my problem.

Buddy of mine always says that nearly all of the best tools out there are just replacements for spreadsheets. Could try starting there :P

Find a pain point and fix it. Like others have said, look for problems, not solutions. This is easier if it's your own pain or something you're already close to.

For my own project, I hate dull repetitive work (like laying out photo books), couldn't find another solution that pleased me. Made my own tool, use tool made my own book => commercialise tool.

Watch people around you and see what they are struggling with. Sometimes people fight against things that should help. So, it's always an opportunity to find an idea.
But a real problem, nowadays, many ideas are already implemented. I would say, almost any solution can be found for free. But are these solutions convenient? Do they really help? It's always a question. Ask and see if there any gap to be filled.
And, just to get some ideas you can subscribe to my free email series about SaaS applications. A free idea of SaaS app into your mailbox, every Friday.
Based on real pain points, taken from life.
https://www.caravelstudio.io/saasideaweekly

Consider someone else already having the idea as validation of you having a good idea. Sometimes it's completely ok to look at another product and ask yourself, "what would I do better"

Case and point, Skype was never the end all be all of peer to peer communication, business or otherwise. Slack, Discord, Go-to meeting, and yes, even Skype all have their own spot in the market after years of development.

Instead of just Myspace or Facebook, you have Instagram, Twitter, Reddit, Tumblr and IndieHackers! Don't be afraid to get out there and fail. You'll learn so much more from trying than giving up.

TlDr; Don't wait for a magic bullet "new" idea, be brave and make old ideas better!

I am definitely not a runner, but I do occasionally like to take walks or jogs without technology. When I am out in the world with my face not buried in a devices is when many of my ideas come to me.