Piece of paper
Piece of fruit
Apple cider vinegar (don't use regular vinegar, it repels the flies)
Step 1: In the bottom of your glass put some apple cider vinegar, syrup, honey and sugar. I didn't measure anything, but I guess that I did about 1 t of the sugar and 1 T of each of the other ingredients. You don't need to be exact. Then pop it in the microwave for about 15 seconds to help the sugar to dissolve. Then toss in a little bit of cut fruit.
Step 2: Take the piece of paper and bend it into a funnel shape so that you have a teeny hole. Tape it along the edge so it holds this shape. Place the funnel into the glass like so and tape around the edge to seal the cup to the funnel.
Step 3: Place the trap near where you're seeing a lot of them (usually by the fruit bowl or a sink) and wait for it to work it's magic. The flies will be drawn to the smell and go down the funnel, but can't get back out. Yay!