It is advisable to grab an old CD and push it half way into the CD-ROM drive until it is visible through the hole marked in the first picture. This will prevent the part that has to be removed in this step to fall down into the unit, with the risk of damaging the laser.

The metal plate slides forward as shown in the first picture.

Use a pair of tweezers to very carefully slide forward and then lift up and remove the metal plate together with the white plastic disk, if they fall into the unit you are going to have to take it apart completely (using this guide).