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You can squirt some directly into your prewash water (I do three squirts which I think is about 1/3 of a cup), or you can spray it directly on the dirty diaper after you've cleaned the poop off and let it set a bit before washing. I've tried both ways and I like spraying directly on the diaper, I think it works better and uses less.
I agree with the last post. I had some major stink in my diapers so i ordered the gallon jug of bac-out online. I just dump a little bit in my pre-wash (probably the same 1/3 c. as above). I had to do this for 2-3 wear & wash cycles until all the diapers no longer smell. I'm not using it now because the smell is under control. It was amazing because I tried everything else with no luck.