Almost everyone agrees the general workflow is to tack down two opposite corners with a soldering iron and when alignment is verified use your favorite technique to solder the remaining pins in place. But getting the IC perfectly aligned (and keeping it there while tacking the corners) is tricky.
Two things I figured today:
2. Next while keeping the IC pinched firmly in place I used a washing line peg to clap it in this position. I found the peg applied enough force to keep it reliably positioned, yet it was still possible to nudge it slightly should the alignment need adjustment.
I then tacked two corners, removed the peg and soldered the rest of the pins.