Modding is easier than one would imagine!
To clarify this, when you create a very simple mod through the AssetManager, it is always going to be listed in Custom Game, although as long as you won’t create a new world for your mod, you are still going to play the normal Grim Dawn campaign you already know.
If you do not want to play in Custom Game (since you won’t be able to use existing characters without some work), you need to merge base files with your modded/new files.
But before you do this, you actually need to create the mod and test it in Custom Game. You can use the mod I uploaded (tempMod.zip, not the other download) to see how I added the recipe.
I’m confused. The download called Shards to Crystals Blueprint does add one new default recipe to the blacksmith that lets you turn one Aether Shard into three Aether Crystals (that’s what you wanted?). It is already merged with the base game, so no need for Custom Game.