Colin Rowe, the maker of the diagrams, added on the mailinglist
some more text to help people through the last stages.
Here you find his text.
Figure 20 shows the start of the formation of the tail,
and also the start of the flippers. The valley folds on the top
layer of the paper are correct. In forming them, the proto-flippers
stand vertically, and are then folded over as in the diagram.
The head is formed by the action of the two top layers being folded
down either side. This should cause the top layers on the 2D "head"
to move outwards and back towards the centre, becoming 3D. Each
side of the head will have a pyramid of paper pointing inwards.
No new creases are made.
There is an error in figure 27, in that the paper by the head
is the wrong colour; this is corrected in 28. In the formation
of the flippers the first pair of swivel folds are flat, while the
second set are designed to give the flipper volume, so the valley fold
is flattened and the flipper is shaped to be "spoon-like".
Figure 29 - show's the result, turn over and spread the carapace.
Figure 31 - sink the two pyramids, this also takes the next layer to the
side of the head.