To customize your gear using Asphyxia CORE, open the Asphyxia CORE WebUI by putting http://localhost:8083/ in your browser's addess bar.
On the main page, look to the far left, and click "IIDX" (or "IIDX@Asphyxia, depending on what version you have)
Once in the IIDX menu, look to the right to see what customization options you have, and select the ones you want from the dropdown menu.
And that's the whole reason this website exists, because there are no previews for the customization options, it's annoying to have to look at all of them one-by-one. So I hope this website can make your customization experience a little bit better :)