Open the last folds that you have made. ( as shown in the first picture )
Now comes the tricky part. Measure 1 cm from the corner, to the direction of the center. ( I used quite alot picture to make it clear. )
Fold it. ( as shown in the picture )
Then pull the corners till the second folding line. ( as shown & tagged in the picture )
Then make the corners meet at the center.