Please have some patience until the little film at right is loaded completely.
At right you see how the Brompton is changed into a small parcel in no time.  Often the time here is relevant: Maybe your train is leaving in a few seconds?
The steps for folding:

1. Fold the rear wheel assembly
2. Unscrew the hinge clamp on the mainframe and fold the front wheel assembly backwards
3. Unscrew the hinge clamp of the handlebar assembly and swing the handlebars down
4. Fold the folding pedal
5. Release the quick release lever of the seat pillar, move the saddle down and fix it again.

It is important to move the saddle down at last. The seat pillar locks the whole folded package. If the seat pillar is moved down too early, the rest of the folding is blocked.
By the way you will never find somebody locking his normal bicycle (to a fixed place) quicker than you fold your Brompton!