So, before you start customizing your headphones, you need to gather up the following:
-Textile paint
- 2 paintbrushes (1 large, 1 small)
- Clear nail polish
- Plain headphones

You may be doubting the textile paint for painting onto plastic, but after applying your clear nail polish, it'll be alright.
Start brainstorming your design before painting everything on, so that you can avoid your paint from drying.

Step 1: Setting Up

Before painting on your design, paint on your background, this will also serve as a primer, so that you can easily paint on other colors. As much as possible, make sure your paint is at just the right consistency that it will be smooth enough to spread, but thick enough to cover the underlying color.

Step 2: Putting Your Design

If you want to, you could sketch your design before painting it on, otherwise, you can just wing it, and see how it turns out.
When putting on your design, start with the outlines, but do avoid the small lines that'll go in spaces that'll have to be colored in later.

Step 3: Color It In

Start putting in your colors after letting your outline dry. Even if your coloring overlaps the outline, that'll be fine, you can always just go back to that after you've finished coloring. Make sure to use the small brush so that you can avoid any mess.

Step 4: Finishing

Start adding your final details after your colors, so you can start wrapping up. Let it all dry up, and now we will use the clear nail polish. This will serve as your sealant. While applying the polish, be careful, make sure that you apply it lightly. Dragging the brush heavily above your design will cause some parts of it to chip, and it will blend with the other colors, making your work messy.

Let the polish dry, and your all done.