I'm new at this and have tried boxed fondant and love the outcome, but i wanted to try and make my own so i used the mmf receipe i found on this site. i let it sit overnight, & it was very hard so i put it in the microwave before rolling it out. but it was not smooth at all, kinda like old playdough??? and looked horrible on the cake, so i just tossed it! i don't know what i did wrong, i did use most of the sugar, could that be it??? should i try using less sugar?? help me!!! i need advice!!

It seems that most likely you had an air leak somewhere. After making MMF I ball it up, coat it with a generous amount of shortening to hold the moisture in, and wrap it in two coats of Press N Seal wrap.
But one thing I've found is that not all marshmallow brands are created equally. When I use Kraft Jet-Puffed ones, the fondant turns out perfect. I tried my local grocery store brand and the fondant was basically unusable even though the texture and taste of the marshmallows seemed the same before turning them into the fondant.

If it feels dry, add more Crisco. If not just keep microwaving it. I do mine in 8 - 12 second intervals, depending on how dry it is and kneading it in between. Then roll it out onto shortening. I tried PS and corn starch and they seem to affect the texture eventually. Keep adding crisco as needed if it gets sticky while kneading. When it is TOO soft, I roll it out and let it sit for a few minutes before handling it. MMF is all I use. Everything else tastes horribly and or packaged.