How do I fill a template like this in GIMP?

you open it in GIMP
make sure you're on the layers view on the right window.

drag the new image into the main window - it will appear as a new layer.
you can re-arrange layers by drag/drop in the layers view to determine which will be on top. the small eye beside the layer name toggles visiblity.
position and resize the image below one of the template openings.
select the image layer, make sure it has an alpha channel (if the name is in bold, it hasn't - right-click and add alpha layer).
roughly box select the visible image. invert the selection (CTRL + I). delete the superfluous parts (DEL).

rinse and repeat.

>>54761
Thank you.

>>54772
Intended for >>54767

>>54767
>select the image layer, make sure it has an alpha channel (if the name is in bold, it hasn't - right-click and add alpha layer).
>roughly box select the visible image. invert the selection (CTRL + I). delete the superfluous parts (DEL).

Or you can just use the crop tool (scalpel) and check "Current layer only" to just crop the layer without worrying about transparency or selections.