Step 1:
Mountain-fold in half, then valley-fold over the line on the right.

Step 6:
Prefold the edges to the centerline.
Then unfold the folds of step 3 to 5

Step 5:
Fold the edges to the centerline.

Step 2:
Prefold over these two lines. The valley-fold is in the middle of the part on the left.

Step 4:
Prefold in half over this line.

Step 3:
Mountain-fold over this line.

Step 10:
Pull the part on the left outwards, see arrows

Step 8:
Tape or glue both edges together.

Step 9:
Repeat steps 7 and 8 three more times.

Step 7:
Press both edges against each other by folding over these three lines. Use the prefolded line of step 2 as guidance.

Step 16:
Press the bottom further downwards

Step 15:
Repeat step 14 on the seven other same spots of the model.

Step 12:
Make sharper folding lines

Step 11:
Valley-fold over these two lines, start with the upper line.

Step 14:
Press the spot indicated by the arrow downwards by folding over the three purple lines.

Step 13:
Tape or glue both parts together, see arrows.

Step 17:
Place the bottle on a table and press the bottom flat from the inside with a pencil, ruler, knife or something else which is long enough to reach the bottom.