Soldering heatsink pad on bottom of IC
What I do for prototype boards that I'm soldering by hand is to put a large hole in the pad and feed solder into it with the soldering iron. 2 mm works well.

Solder the other pins first, so that the chip is fixed in position.

The flux in the solder will be sufficient.

Number of holes depends on the size of the pad. One is usually sufficient.

You need a good soldering iron with plenty of heat, I use a Metcal.