Prepare some sterile water (boil and cool), add 1 teaspoon brewing sugar to half a pint, open bottle, pour out beer, add sugar solution, plug neck by stretching over a new poly bag and securing with rubber band, swirl, place in a warm place, drink the beer, after 24 hours swirl again, if poly seal "inflates", you have succeeded. But beware, some makers add a different yeast to prime the bottles.