Step 1:
Get a square paper. White side up and fold into half. Unfold. You will find a creased line.

Step 2:
Fold the upper right hand corner to the creased line. Make sure your fold goes through the lower corner. Unfold.

Step 3:
See where the first two creases intersect? Fold the upper right hand corner to that point. Unfold.

  
Step 4:
Fold point A to point B to get something similar to the second picture.

Step 5:
Then fold the two top corners down.

Step 6:
Unfold and turn over to the colored side.

  
Step 7:
Fold the right side over at the creased shown.

  
Step 8:
Open the layer underneath of point A and fold to the right.

Step 9:
Do the same thing to the left side. You should get similar as the picture.

Step 10:
Fold the top ends downwards and bottom ends upwards, to have a smoother curved ends.

Step 11:
Fold point A to line B.

Step 12:
Fold again so the sharp bottom looks curved.

Step 13:
Turn over, and you have a mushroom!