While the gelatin is soaking, stir the sugar mixture 2-3 times, very gently each time. When the sugar has dissolved, remove the lid, attach the candy thermometer to the side of the pan, and turn the heat up to high (medium high for induction). From here on out do not stir or otherwise bother the sugar. Keep an eye on the thermometer; you want to boil the sugar to 248-250 degrees F.
Here comes the exasperating part–but with practice it will become less tricky. Quickly undo the whip attachment and set it aside–accept that you will lose some fluff sticking to it. Using that very sturdy spatula that has preferably been dredged in powdered sugar, quickly and firmly scrape the marshmallow fluff into the prepared baking pan. It will astound you how fast it sticks to everything! When I first started making marshmallows I saved that sticky fluff like it was gold and spent long minutes scraping it into a cup of hot chocolate. Nowadays either the kids lick it off or I just throw it in the sink and accept it as lost.