Lower & upper impressions (with single use trays).  N.B.All impressions should go through a disinfection process before casting takes place.  This is usually performed in the clinic / surgery.
Upper and lower rubber base formers.  Note the upper has a pointed anterior section as opposed to the lower rounded anterior section.

The mixed plaster is then gradually applied into the impression (a vibrating machine can also be used to encourage the plaster to flow into the impression) by tapping the impression on the side of the mixing bowl.

N.B.  The plaster is then removed from the impression (tapping the plaster out) and reapplied in the method mentioned above.  This minimises the risk of airblows on the teeth / fitting surface on the set model.

At this stage, some techniques allow the plaster to set in the inverted impression before mixing up more plaster for the base.  It is currently common practice to cast the impression and make the base 'in one' method.

The impression is then placed on top of the rubber base former and the excess plaster wiped away using the fingers.

N.B.  When casting the lower impression, ensure the lingual section of plaster is removed before setting to enable ease of removing the impression.

Once the plaster has set, remove the rubber base former, then ease the impression in the buccal area on both sides and gradually lift the impression from the model using a plaster knife. 

N.B.  Extra care should be taken when removing a partially dentate impression as the risk of breaking teeth / incisal edges is greater.