The service I will be describing today will be http://www.gixen.com/index.php
This service has done wonders for me and I highly recommend it, but there are plenty of other sniper systems out there that you can look into if you are not satisfied with Gixen.
First thing you will want to do is log-in using your standard ebay id and password. Then you will be brought to this screen
Your screen will not be populated with the items from your watchlist yet, though, so go ahead and click the "import watch list" button and your watch list from ebay will be brought in.
If you decide that you want to set a bid that is in your watch list, just click "Edit" next to the item you want.
This will highlight your selection in red. Then you will want to scroll down to where it says "Your bid:" and enter the maximum you would want to bid on the item you chose. Do not forget to factor in the price of shipping, when you are setting your bid.
And that is pretty much all you have to do. Gixen is intelligent in that it will bid just over what the bid is at in the last 3,6,9, or 12 seconds meaning that it will try and score you the item you want at the lowest price possible as long as it does not pass the bid price you set.
Good luck and happy sniping