I think I can help; I altered a pattern like this once for myself.
In this picture the black lines are the original pattern.
So, you would extend along the red lines
down the sides to lower/extend the waist. The middle red lines show that the three pieces that make up the front are all extended (at least, it looks like three pieces in the picture you posted...). Same for the back.
The green lines
are if you need to increase the width to go over your hips. If you do this you are then changing the width of the bottom edge of the bodice (in purple
The purple line would be the measurement of your hips where it will sit divided by two, plus seam allowances.
The bottom edge of the bodice and the top edge of the skirt
should be equal to each other so they match up when you sew them together.
The skirt is increased to the light blue lines
to keep the fullness of the skirt. You can just cut parallel to the original pattern, but outside of it as far as you need to.
--> all of this is the same for the back. Be careful if there is a zipper; extending the bodice is going to change how far down the zipper will reach.
I hope that helps; good luck!