Do the search just like you would for an active item. When the results come up, look at the left side of the page, you'll see catagories, and under that are guides, under the guides are options. Click in the box next to completed items, then click the show items button.

or...

click on the advanced search link in the upper right corner of most ebay pages, it will take you to a page where you can define your search, including telling it to search for completed items

For completed auctions, it will not search the descriptions...and it might only search the last 30 days.

BTW...Welcome to the club!