I only microwave for about 45 seconds, then take out and stir with a crisco covered spatula. If they are still not soft enough then microwave for about 20 seconds. You don't want to overdo it. They will still look like marshmallows when you take them out to stir them.
I've never made a well, but that would probably be a good way to do it. I've just added a little sugar at a time stirring it into the marshmellow as I add it.
Try adding a little water, a little bit at a timewhile 
kneading until you reach the right texture.
I am no expert and certainly not as good as alot of people on this site, however, I did find a website with some great directions and PICTURES (step by step pics). If your anything like me I need pics so I know what I'm doing looks right. I hope it helps you.. here it is http://whatscookingamerica.net/PegW/Fondant.htm. I'm not sure if it's as good as some of the recipes on here, but it was quite useful for me. I printed off the pages and use them when I make the mmf. Good luck!!
Also, I forgot to tell you that if you make it in advance, you need to grease the ball of it with a thin layer of crisco, wrap it in 2layers of saran wrap the I usually put it in a resealable bag. I think the crisco softens it up a bit when you start to knead it again. Also, VERY important, only use 10x sugar for me it's NOTHING but DOMINO!! It truly makes a difference. Just a suggestion.
http://whatscookingamerica.net/PegW/Fondant.htm try this link instead as the other one does not work right. good luck
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