How do I make carbon dioxide for a mosquito trap?
Vinegar and baking soda will produce carbon dioxide very quickly; the reaction will be over in a matter of minutes. And dry ice requires special handling and going back and forth to the store many times to procure fresh ice.

A better way to continuously produce 1 to 3 humans' worth of carbon dioxide over a period of about 3 to 4 days is via (yeast) fermentation. 

You will need a 5 gallon bucket with a lid. Drill a hole in the lid of the bucket and connect a length of hose long enough to reach from the bucket lid to an area adjacent to the zapper; most standard garden hoses are 5/8-inch in diameter; you will want the hole to be slightly smaller than the hose so that the hose fits snugly in the hole. Don't put the business end of hose too close to the zapper, because foam from the fermentation process might travel up and spill out of the hose (onto the zapper!). You will want to set the bucket on a piece of wood or insulation to prevent the ground from cooling it. It may be helpful to paint the outside of the bucket black to help it absorb heat from the sun.
You will need 750 grams (1.65 lbs) of sugar and a pack of active yeast. Mix the sugar with 1 gallon of hot tap water. When the water has cooled to around body temperature (feels neither warm nor cool on your forearm) add the yeast and close the lid.