If the ground is not level and one end of you panel has a gap from the bottom of the board to the ground you can do one of 4 things to reduce that gap:
a) Take the earth that was dug out of the post hole and spread it along the bottom underneath the base board where the gap is.
b) Dig out some of the earth from the side of the base board that is closest to the ground until the base board is level and there is little gap underneath.
c) Shorten the space between the posts so the level step sections are shorter and the gap can be covered more easily by these two methods.
d) If there is a large gap under one side of the panel, keep the flat board laying on the ground, level your base board on top of this board and measure the gap on the wide end between the flat board laying on ground and the bottom of your base board and the length of where the base board touches the flat board. Then remove the base board and create a wedged board or boards to be fastened to the bottom of this base board using screws and glue. Once this new base board has been created and dried insert it into the bottom as your base board again and it should now be level on top and close in the gap on the bottom.