Get the correct Demon Hunter artifact equipped
Unlock the hidden appearance for your spec
Check the current source for the appearance unlock method
If it is a drop, farm the required mob or boss repeatedly
If it is a vendor item, buy it from the correct vendor
If it is a quest reward, complete the required questline
If it is a world drop, farm the listed zone or enemy type
Use the appropriate spec when farming or turning in the requirement
Increase your chances with bonus rolls, if available
Keep the hidden appearance in your artifact appearances tab after unlocking it
If you want the easiest route, use the fastest available farming method for the current unlock source