Found this shared by Hobby Mate via Gundam Tutorial group. The whole process was made by Indra Bayu. Thought we'd share this to you guys to help you for your everyday GunPla building knowledge.
1. What you need to do this are matte spray, pencil, cotton bud, and the part of GunPla you want to shade.
2. Coat the part you want to shade with the matte finish coating spray.
3. Rub the pencil to the edge of the part you are shading
4. Check if the sides are darken enough
5. Smudge the lead you have applied to get a gradient effect that would look like a shade on your kit.
6. If you did it right, the result would look like this! pretty neat huh?
7. Do the process on other edge of the part.
8. Do not forget to coat it again to avoid the smudges from fading.
9. The same technique are used with the panel lines, but this time you can use cotton bud for more controlled shading.
10. You can do this by powdering the lead by rubbing it on the middle of the tiles or just by simply using sand paper.
11. After scratching it, you can take the powdered lead with your cotton bud.
13. You can start rubbing the cotton bud on the part of GunPla that you'd like to shade.
14. see that? it's very nice yes?