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