Add a couple spoonfuls each of honey and dish soap. There's no exact "right" amount, so don't worry about measuring. Give your solution a gentle stir to combine the soap, honey and vinegar, without making bubbles.

You can use the trap as-is, with an open lid, for optimal results. However, if you're worried about spills, take a lid and poke holes in it. This will allow the fruit flies to smell the vinegar and get inside the container, but prevent any major spills.