in this guide I'll use the RPGM2k3 charset template. When you start aseprite open up a template to begin with.

Gamemaker has a well done spiting tool included in it but what it really lacks is layers and that can make spriting difficult, especially the animations. Working with templates is easier with layers.Ever tried to work with templates but you keep drawing over them or mesing it up in a different way? Layers is your friend here.