>>918771
Well depends on the tools and resources you have available.

The electronics are easy. 4(3 if your cheap) wire cable a stereo 3.5 mm jack (for most PCs or a 5mm if you use it for guitar amps or high end recording stuff)
And two decent 16ish ohm speakers (if you don't want to use a preamp). This sets the quality of the sound depending on how much dosh you want to burn.

The rest is just the casing. Now /mu/ would argue that this is a very important part. As here you will get sound leaking. Witch is a problem if you do some recording and you get bass and treble bleedout. This is pseudo science for me since I use a big ass 800W sound system for my music.
But from what I can gather you don't want any holes or gaps for your sound to "escape" form in your speaker casing. You could cast there like they did in WW2 shape some sheet metal to a shape you like and than fir the speakers in it or if you have a friend with a 3D printer you can be a baller.

Next up is the cushions for your ears. Well cut some sponge in to donut shapes and wrap it in ideally leather but use whatever you like. Use a stencil to get the cuts.

The bridge between the headphones may carry a cable that connects the left to the right one of you can do it old school and have the wire split to each headphone depends on what you are looking for.
The bridge in between them will then need to be built according to that decision. you cant make it with tin snips and a sheet of thin aluminium and shape it out with a hammer around a cheap ball (or if you have a spare anvil lying around use that)
Put that all together and you'll wish you spent 100$ and both a decent pair of Sennheisers

>>918771
I've made a couple of pair of headphones, but the quality isn't really that great, its barely passable, even 30$ store bought ones perform better than the ones i made. The problem is, its hard to obtain low impedance wiring and sensitive speakers