This is for breastfed babies, mine isn't formula fed so I don't know if this technique works for that as well. The green shirt is a fresh stain, the pink and blue shirt is a few days old.
You need 2 things: Fels-Naptha and Oxy Clean. Fels-Naptha is $0.99 at Walmart.
The fresh stain:
First: rinse it out as best as you can. The sooner the better, as you can see:
Rub Fels-Naptha on both sides of the article of clothing. The soap is yellow as well so keep track of where you've rubbed:
I rubbed only the right side and you can already see a difference:
Finish rubbing it on the entire stain.
Next, press wet fingertips into the oxyclean
Rub it into the stain. There are blue grains and they will break open and spread blue over. That is fine, it will wash out.
Again, do this on both sides of the clothing. (This pic has the oxyclean rubbed only on the left side)
This is a stain that's been dried for a few days. I applied the same method as above to this:
After treating it, I set it on my washer to dry (so I don't forget to throw it in with the rest of the clothes) and then I put it in the laundry when I do it next. Fresh stains can be washed immediately. I haven't ever treated an old stain and then washed it immediately so I don't know if that would work.
After being washed and dried normally:
Both are spotless!!
I've never had to bleach or throw away a onesie. I hope this works as well for you as it does for me!!