So I found this technique from Youtube (it's in Bahasa Indonesia btw but it has some translation baloons):
And so I tried and ended up finding my own way. I would ask you to choose which do you prefer seeing in a comic with manga style between the four images I developed:
Outline with gaussian blur
Outline with gaussian blur with bitmap conversion
Outline with threshold
Outline with threshold with bitmap conversion
If it's hard to distinguish between the bitmap-converted and not, you can just choose between the gaussian-blur-outlined and the threshold-outlined. Thank you!