This can be very easy, what you need is a bowl of water, some fallen leaves and a shaded corner out your back.........
Place the water in the bowl in a quite corner of your back and place leaves on top of water leave the water for a week or two..........
Hey presto ! you will have bloodworm, the flies come down and lay the eggs under the leaves and it all starts from there but be patient because this can take some time to get underway, there will not be greate numbers to start with.
Strangly enough i found this out at the owl center i work at, the wild tawny owls bowl doesn`t get changed very often so is left for weeks on end, when i went in there were loads of then and they just kept on mulitiplying.....try it yourself
Just keep topping the water up now and again....the chlorine doesnt bother them at all for top-ups