SMT soldering for QFN package without legs
I certainly would not use a microwave oven here.  That runs the risk of damaging your board, components, and also the oven itself.
I've had good success with a &quot;rework reflow&quot; process where you do the following:

Apply suitable flux to the pads and to the pins on the device.
Apply a small bead of low temp solder paste to the pads.  It's OK if there is some between the pads.
Place the part using some glue under the center if it won't stay put.
Use a heat gun that produces a high enough temperature to flow your solder paste to the  pads until the solder reflows.

Alternatively you can use a hot plate or a toaster oven to heat the entire board but I've had the best luck with the heat gun.