The best advice I can give is to not pour all the 2nd half of powdered sugar in at once. I usually put in about 1/2 of it, then slowly add as needed.
Also, if it hardens you can put it in the microwave to soften it back up. I add the coloring to it once it is warm and it is much easier to knead in (don't forget to wear gloves or you'll end up with colored hands & add some shortening to them to avoid sticking) Just put it in for seconds at a time. Be careful, it can get hot really fast.
And... CRISCO CRISCO CRISCO!! I can't stress this enough! It will totally change the MMF to something that you can easily work with! If you are doing a full batch knead in about a tablespoon and it will become supple, silky, elastic, and easy to use!
Oh, and if you are coloring... add it to the melted marshmallows and stir before adding PS... so much easier!
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