The process is going to be different depending on which game engine you use (Unity, Godot, GMS, Unreal, etc). You should search for tutorials about something like a "paper doll system" or "layered character animations" within your game engine. You basically need all layers to animate in unison, showing the same frames at the same time. If you have trouble with something, you can try joining the Mana Seed Discord server (linked at the bottom of the page) and ask other devs who are using your game engine.