You don't "apply" a Harmon Target. Products are generally tuned to it. This also depends on the type of product. For example, a headphone is tuned differently than a IEM due to how the ear and ear canal modify the frequency response.

Another thing to note is the Harmon Target varies with volume, so a single curve is only suited for a single listening level.

If you want to tune your headphones, I suggest listening to a pink noise track. Run the track at normal listening levels. Then work on the EQ. Usually as set 1kHz at the 0 point and then scale everything else from that. Your goal is to move up and down each of the other frequencies to tune the relative loudness of that other frequency range. You want 2kH, 4kHz, 250Hz, etc. to all sound the same loud as 1kHz. If you're too high in volume in an area, it will sound over pronounced, dominant against the rest of the frequencies. If you are too low in volume in an area, it would sound missing or sucked out. There will be kind of a sound void there where you don't really perceive that frequency range against the rest. You want to move the sliders up and down until each frequency range sounds equally loud, equally present as the others. One note, this test is slightly sensitive to tonal slope as in you can end up with a "flat" sounding setting that's overall tilted bright or warm. The "tuning" especially tuning just the next slider over can sound good/right along a slope. You'll have to kind of step back and compare broader distances too, for example 250Hz with 8kHz and make sure those are in balance with each other too. Play around a bit and spend some time dialing it all in. Also, I suggest doing this more than once. Tune it, and then leave it for a few days. Then come back and retune. Your perception of sound changes over time, and you are also learning what to listen for. It might take a few attempts to really get it settled in well.

Secondarily, the above is only a test of raw loudness. Perceived tonal balance afterwards highly depends on exactly how the headphone produces sound. For example, let's say the headphone uses a driver that is tight and crisp in bass response. The notes are not thick or weighty, but the volume level is there. Because the notes are thin, the overall perception is the headphone will be a little bright even though the actual sound volume is there in the bass. You might actually have to EQ up the bass a few dB to counter the lean bass notes. It can work the opposite way too. You might have harsh or aggressive high frequencies, and as a counter, you might desire to knock down that aggressive high end a touch to make it less edgy. Manufacturers do a lot of this trickery to tweak the end result into a perceived presence that might be realistically quite unbalanced and colored. It might just be the ideal compromise for the characteristics of the driver and product design.