How do i solder qfn package using soldering iron?
I normally put some solder paste on the pads, place the QFN on the board, and reflow with hot air. With some trial and error you learn how much solder paste to put, but you really need quite little.

I have seen it done with the soldering iron as well, by following the process above, but instead of  heating the device with hot air, you hold the device in place with tweezers and heat the pins one at the time with the tip of the soldering iron.

You can find plenty of videos on how to do it on Youtube.