This is the easiest new item to be created because you DON'T need to edit the resourcepack and upload it, so it's really fast to be done also when the server is running.
To create custom armors you just need to follow these simple steps:
1) open customHelmets.yml (you'll follow the same steps for other armor pieces files: customChestplates.yml and customBoots.yml and customLeggings.yml )
2) Copy one of the already created helmets in the file
3) Now have fun, you can edit every attribute as you like. To edit colors you just need to generate the HEX value using this simple tool. Then copy it removing the # character.
4) Now just reaload the server or reload NewItems in order to make your item craftable ingame.