Step By Step Guide to Leaving Feedback on eBay
There are several ways to leave feedback:

One is to go back to the closed auction, and click on the icon that reads "Leave feedback to seller" or "Leave feedback to bidder". By leaving feedback in this manner, the transaction (auction) number will show up in the actual feedback. 

Go to the bottom of ANY ebay page and click on "Feedback Forum" Once there you have several options. The easiest is to click on Leave Feedback About An Ebay User - see all pending comments at once. Clicking here will show you all auctions (with the past 90 days) in which you've participated, and haven't left feedback for the seller/buyer.

Go to Search page (from that task bar at the top of ebay pages), then click on "find members" (a sub head in the task bar under Search). Write in the user's name under "Feedback Profile" and you will be taken to the user's Feedback Profile page. Once there, a line or two under their eBay ID card, is the option to "Leave feedback for this user". 

Another way to access this page is to search the auctions bid on (or sellers past auctions) on the main search page. This will give you the same option. 

Click on the number next to the user's ID, and it will also take you to their Feedback Page.

Feedback Stuff you Need To Know
Before you bid on an item on eBay, be sure to check the seller’s feedback rating by clicking on the number next to their name. Even if you see a large amount of positive feedback, check to be sure there are not a growing number of negatives in recent history. On eBay, you’re only as good as your last few transactions. If you see a lot of wonderful feedback, but a growing negatives, beware.

Take a breather before you leave negative feedback. EBay won’t remove feedback if you change your mind or over react to a situation.

If someone gives you positive feedback, respond. If you receive negative feedback and feel your side of the transaction is worth telling, give your reply in a neutral tone.