In hope of simplifying the process, I followed some 3rd party instructions I found for hacking together your own referral URLs, but they didn't work. I asked Amazon instead, and here are the official instructions:
If you would like to link directly to an item's detail page using a simple text link, use the following format to construct your links:
To make this link functional, replace "ASIN" with the 10-digit ISBN or ASIN of the product, and replace "your_Associates_ID" with your Associates ID or tracking ID. However, you would not be able to link to a search results page on Amazon.com.
If you would like to link to the Amazon.com homepage, you can use the following format:
To make this link functional, replace "your_Associates_ID" with your Associates ID or tracking ID.
An item's ISBN/ASIN is listed on the item's detail page. For example, follow the link below, and then scroll down to the section labeled Product Details to find the ISBN (0439434866) for "Harry Potter Paperback Boxed Set."
I don't make much money from referrals, but I do enjoy taking my payments in the form of Amazon gift certificates and buying myself a little something from my wish list every now and then.