Step 6:
Tape/glue the parts indicated by the green squares together.

Step 5:
Prefold the edges to the centerline.

Step 2:
Make papers of various length, see picture.
Fold over the red lines, they are placed a bit random.
Make less of these folds on the shorter papers.

Step 4:
Prefold in half over this line.

Step 3:
Mountain-fold over the red line, then turn the model.

Step 10:
Hold the model on the place indicated by the yellow dots. Then pull the part on the left of it in the direction of the arrows.
This is just an example, vary on each part of the cactus plant.

Step 8:
Press/pull/push these edges downwards with your finger.
Repeat on each side of the model.

Step 9:
Fold all four edges on top downwards, see red lines as example.

Step 7:
Press both edges towards each other.
Make these folds on each side of the model.

Step 1:
Prefold a square paper in half, then cut along the prefolded line.
Tape/glue both pieces together to make a long rectangular folding paper.

Step 16:
Prefold over these two lines.

Step 15:
Squash-fold the flap.

Step 12:
This is a standard paper with a prefolded squares pattern for making tiny Origami papers.
Prefold over the four red lines.

Step 11:
Make enough of these cactus parts for one completed plant.

Step 14:
Fold one flap straight upwards.

Step 13:
Valley-fold over the two lines at the bottom right. Then mountain-fold over the two diagonal lines in the middle. And valley-fold over the two lines at the top left.

Step 21:
Bookfold one layer to the left

Step 19:
Valley-fold the triangular flap upwards.

Step 20:
Repeat steps 15 to 19 on the other three flaps.

Step 18:
Press/push these edges downwards.

Step 17:
Valley-fold over the red line.

Done!

Step 22:
Pull the petal downwards by making an inside reverse fold over the three red lines.

Step 25:
The flowers are glued to the cactus parts.
The cactus parts are placed in a pot which is filled with wrapped papers. Finally, small dark green paper balls are placed on top of the wrapped papers.

This DIY-video by Joost Langeveld also has instructions for making the cactus plant with tiny flowers:

Step 24:
Make as many of these tiny flowers as you want...

Step 23:
Repeat step 22 on the other three petals.