Most of the instructions are in the Discount config screens themselves, rather than in our help pages. It may help understanding to know that our discount settings were originally conceived as "virtual coupons" that could be sent via email; as such, the idea was that you simply make up a coupon codename like "WinterSpecial07" and configure that in your cart, so anyone who has that code (say, from a promotional postcard or email newsletter, or simple word-of-mouth) could type it into your cart screen to redeem the coupon. That expanded into single-use/full-price coupons, so someone could, say, buy a unique mp3 coupon code at a show and use that to discount to zero their online "purchase" of that mp3 download. Finally, we combined this with our "Send Stored Codes" product setting to create a sort of Gift Certificate.
For single-use discount codes, rather than setting a generic coupon code that anyone could redeem any number of times, you'd want to generate a bunch of unique "serial number" type of codes that are no good after they're used once (you could probably whip these up in Excel, or find a site that has a free code generator you can use). Then you would go into your Seller Admin > Cart Discounts & Gift Certificates to create each GC discount amount. First, give it a name (like say, GiftCert25 for a $25 cert.) then on the Discount Code line, paste-in your list of unique serial codes, separated by commas. The remaining settings should be self-evident, just remember to set Maximum Use Count to 1. to make each of your unique codes a one-shot deal.
Then you would go into your Seller Admin > Add New Product and create a product with the Send Stored Codes setting marked, and the same price as the discount amount you just created. When you get to the product config screen where you would paste-in your codes, paste the same list of unique serialized codes you provided when setting up the discount. After you're done adding this product, go back to your Cart Discount screen to start setting up the next "level" of GCs you're going to offer and repeat the process above with a fresh list of unique codes.
When someone purchases these GC products, it will email them as many codes as they ordered and remove those codes from the product list (our system notifies you when you're down to your last 5 codes for any product). The buyer can forward their codes to the gift recipient (even write them on paper or in a greeting card), who would then redeem them by typing the codes into your cart screen when they're shopping on your site.