One way involves replacing the flash with an antenna on an old camera flash. It's not strong enough to fry electronics, but your phone might need anything from a reboot to a factory reset to servicing if it's in range when that goes off.

I think the difficulty for EMPs comes from the device itself being an electronic, so the more effective the pulse it can give, the more likely it will fry its own circuits. Though if you know the target device well, you can target the frequencies it is vulnerable to, which could be easier on your own device, plus everything else in range that don't resonate on the same frequencies as the target.

Tesla apparently built (designed?) a device that could fry a whole city with a massive lighting strike using just 6 transmitters located in various locations on the planet. If that's true, I think it means it's possible to create an EMP stronger than a nuke's that doesn't have to destroy itself in the process, but it would be a massive infrastructure project spanning multiple countries. There was speculation that massive antenna arrays (like HAARP) might be able to accomplish similar from a single location, but that came out of the conspiracy theory side of the world, so take that with a grain of salt (and apply that to the original Tesla invention also).