I wanna make a Armor GUI

So uhm there's a similar post about this from few years ago and I checked it but it's outdated and I want an explanation for newer versions, so uhm basically I wanna make custom GUI Armor Slots for a Mob I made, like for helmet, chestplate, leggings and boots and I wanna make that you can only put armor parts in, and I wanna make that it shows on the mob like it does for the horse, I hope someone replies asap, thank you forward