I have bought a few items, that have been very slowly delivered.
I contacted the seller, and I do not have any issues, but I would like to leave feedback.
How do I write feedback for the seller?
Feedback is not only voluntary, it is not a metric eBay uses to judge a seller's account.
For a seller with an established account, it's probably not a concern.
After 60 days you can not leave feedback directly.
IF the seller left you feedback when they shipped, you can reply to the feedback from the seller.
To answer your question, "How do I leave feedback", you simply click on the item you intend to leave feedback for, and then click on "Leave Feedback".