Tutorial
The FPS Animation Pack works with UE4 and UE5 Epic Skeletons. This means you can easily add your custom model in a few clicks. First, add your model to the project and assign the SK_Mannequin_Skeleton (for UE4 models) or SK_Mannequin (for UE5 models):
Right-click on your character mesh and assign a new skeleton.

Open up this asset.

Assign your mesh in one of these slots.