just like creatures in my own manga im doing or in the manga of HARUHI SUZUMIYA, i would like to provide you some techniques!!
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1st step: think of the purpose or the function of the character in your manga
2nd: give it a respective name... It should fit its characteristics
3rd: Make its plot!!! And then start planning its look!!
4th: Ask someone to judge it and ask if its already good
5th: add something that make it somewhat fanstasy genre or sci-fi.. LIKE adding wings, adding halo or a gun attacked on its hand.. just like an akuma or the hand in Full Metal alchemist.. MIne, i putted automatic fans and knives attacked on her hands. Another example is the hIME in MAI HIME!!
6th: explore other's work of fantasy or sci-fi and get ome idea. of course dont copy or cheat!!
7th: color and FINISH!!
this are only and JUST tips!! thank you! ^^