Make a waterbomb base. (Don't know how to do that? Look at
this
Origami basics page.)
Reverse fold the inside flaps of the waterbomb base so that the line of
the 2 inside
crease lines up with the slanted edges of the waterbomb.
Here's how the creases need to look...
And here's Step 2 complete.
Turn it around so the point faces you. Now valley fold the top layer down
towards you as
far as it will go.
Here's this step done on one side:
Flip the model over and do the other side too.
Here it is done on both sides:
Valley fold the 4 triangular tabs in half.
Here's one done:
Here's all four done:
Valley fold and unfold each tab along the line of the pocket next to it.
Here's one in its folded state.
Then tuck the tab into the pocket. Here's one done:
Here's all of them done:
Now, with both hands (only one shown here because I was holding my camera)
gently pull the sides
apart so the bottom opens out into a square depression.
Here it is with the bottom opened. As you can see, it's not so neat
yet:
Turn it over and smooth the handle and sides with your thumb and
fingers.
You could also pinch the bottom into a crisper, square form, but I like it
curvier.
And here's the finished product!