This is the tricky part. You should probably use a small pinching clip to hold the sim reader in place, and if you haven't got much experience with micro soldering, I strongly advice using two hands to steady your iron.

Apply a small amount of flux onto the legs and pads, and then touch one pad. At this point, having a microscope comes in handy, because you can see the solder melt.

After the first solder joint is done, you can slightly realign the SIM reader, if necessary.

After that just proceed touching the legs one by one, soldering all 8 legs onto the logic board.

Beware of the plastic on the reader: if you melt it too much it can cause problems with the sim card.