Yeah, we hear you on that one! These QFN packages are pretty hard to solder.
IF it is a qfn without a big ground slug, you can actually put it down with a hand soldering iron and a very fine pointed tip.
If it has a big ground pad in the middle of the QFN, then that will not work. You can put some solder paste on the pads and the big pad, and then either put it on a hot plate, or carefully heat up the underside of the board with the hot air gun. You MUST make sure that the hot air gun is not blasting out too much heat! You do not want the board to catch fire!
One trick I sometimes do is to use wire solder on the big ground pad with a normal soldering iron, and smooth it out as flat as I can. I leave the individual pads for pins dry. I then put the board on a hot plate, carefully put the QFN on top and after the qfn heats up and the solder is melted, I carfully position the QFN and remove from heat to cool off. Then I use a small handheld soldering iron for the pins that need attaching (most QFN pins on a microwave chip are no-connects, and do not need soldering).
Unfortunately, these QFN packages are made of plastic, and heating the qfn chip from the top side will not heat it up enough to get a big ground slug underneath reflowed before the plastic package melts!