Fry pork cubes as usual. (I usually marinade them in soy sauce or just rub the pieces with salt, then fry until brown.)
For the sauce, I use the juice of canned pineapple slices – they’re already sweet and sour. All you need to do is heat it up, add a little bit of cornstarch, probably a tsp or so to thicken it, then add catsup and a little bit of soy sauce for coloration. I usually add some pineapple slices and bits of green and red peppers in the sauce as well.