Don't leave any feedback until the item is in your hands and you have fully checked it over to make sure the quality is just what the seller described, and it works. Ebay gives you 90 days to leave feedback...that's three whole months.
If everything is ok, and you are happy with your item describe what happened; for example "fast shipping" "fragile item very safely packed" "friendly communication" "happy customer" or whatever. I suggest typing out what you want to say before you select positive, neutral, or negative because you only get 80 spaces to work with. If you hit the "enter" button, some browsers will post what you have typed, and there is no way to edit what you wrote. If you don't choose what type of feedback until after you type the comments Ebay won't post it, and you get another chance to fix things.
If the item arrives broken, don't leave negative feedback right away! Most of the time it's the post office's fault, not the seller's. If that's the case, or if you get the wrong item by accident contact the seller by email. Most of the time the seller will fix the problem by sending you a new item (if possible) or refunding all or part of your money. The same process is true if the item doesn't show up at all...sometimes it is being shipped from overseas, and could be stuck in Customs waiting to be inspected. That's not the seller's fault either.
If you have contacted your seller, and still can't work things out, try filing a complaint with Ebay or Paypal (if that's how you paid for it) and they can help by acting as an impartial mediator to try to get you to a point where everyone is happy.
If you've done that, and still aren't happy, then you can leave your negative feedback, but use the comment space to explain exactly what went wrong..."received wrong item, seller didn't respond to emails" "never received item, non-performance claim settled in my favor" or so on.
Don't stress too much about the negative stuff. Almost all Ebay sellers value their good reputation, and will work with you to make you happy.