Hello, your question is not dumb at all!  When you choose your purchases, look for the spot for promo codes and type in your artist discount, click add promo, then right below that is a place that says "Use my commissions to pay for this order and beside it will show what commissions you have available.  If you want to use your commission, just click on that, then it will confirm that you want to do that and then it will subtract it from your order.   I LOVE being able to use my commissions that way!
 
Much success to you.  Hope this helps!