Can someone help me understand how to mod textures?

[CrimsonKnight98]

I want to make a controller button design for Dark Souls Remastered but I don't understand how to design button prompts. Do I draw them by hand? Can I model them in some way? Also, how do I know how much room I have to draw them in the texture files? In other words, how can I determine on the texture file where to draw the buttons so they don't get cut off in game?

 

Any help is appreciated, thank you.