"Flat" by measurement is not the same as "flat" by ear since human hearing is not linear.

If you want to EQ by ear and want a "flat" sound, basically what you want is equal loudness from 10Hz to 20KHz. IEMs are more capable of this than headphones in regards to distortion.

If you want a "flat" sound by measurement (dB SPL) (but doing it by ear) then there should be a gradual roll-off from 1KHz down to 10Hz, a -3dB dip around 1.5KHz, a rise to +3dB at 3.2KHz, then a gradual roll-off up to 20KHz.

I think a happy medium is somewhere in between. Most people like a bass boost and treble is subjective too. Listening level matters. Having a smooth response is key so no stabbing peaks or troughs.

I don't know about "easy." You could try EQ'ing whatever you currently have by ear but that takes a lot of experience and practice. It helps to have a "711" coupler to make measurements. REW helps with modeling. I've EQ'd the $10 Panasonic RP-HJE125 to it for the most part. I've EQ'd the Soundcore Sport X10 to being pretty close. EQing 3KHz+ must be done by ear since it is not accurately measured by coupler and is anatomy and insertion depth dependent. Might take you 20+ bands but it CAN be done.

Use a tone generator to figure out the exact frequency at which this occurs for you (I'm sure you know this). Start with a Q value of 5 and do sweeps across that region. If the adjacent frequencies sound too quiet relative to the resonant peak then try a higher Q value. If they are too loud then try a lower Q value. Do the best you can to smooth it out.

For IEMs I like my target but it actually depends on listening level. That is good for a moderate listening level but the louder I go the flatter I like it. So at louder listening levels I may only like 10Hz to peak at no more than +10dB or +5dB relative to 1KHz and I may only like 3KHz to peak at no more than +3dB or +0dB relative to 1KHz. Its all relative. Loudness compensation linked to volume control helps. The RME ADI-2 series and some receivers have this loudness control feature. This follows the equal-loudness contours of human hearing.

Sponsored by Pfizer.

It is 99% about frequency response and not some hifi/audiophile mumbo jumbo people spew out. I was never a fan of open backs. Please consider IEMs.

HearID is useless and their proprietary target curve is highly suspect.

Try these: https://i.imgur.com/TUG9kT0.png

Or just EQ by ear to your own preference. Only so much you can do, though. The aggressive and uneven treble can't be fixed in app.