By now, all 4 bottom corners are in the bottom layer. There are only 8 corner pieces, and the first 4 are already on the top, forcing the last 4 to the bottom. The first step is to arrange them in the correct positions:
Turn the bottom layer 
Now rotate the entire 

________________________ 

Repeat this 
...until both lower 

R B+ L B 

________________________ 

What the move does: 
You may have to repeat the sequence twice to fix the front bottom corners in place.
For you experts, there are variations to the prior sequence that yield powerful results. There is no need to memorize these, but if you want to save a few moves, here they are:
Sequence: 
Result: 
B R B+ L B R+ B+ L+ B2 
The same 3 corners are swapped counterclockwise. 
B2 R B+ L B R+ B+ L+ 
The (bottom) right 2 corner cubes are swapped. 
B+ R B+ L B R+ B+ L+ B+ 
The (bottom) back 2 corner cubes are swapped. 
The two front (bottom corner) cubes should now be in place. The back ones may also be in place; but if they are not, swap them with the following move:
B+ R B+ L
 
Swap the lower back corners 
You only have to do the sequence once to swap the rear bottom corners in place.
All 4 bottom corner cubes are now in place. The next step is to rotate each corner so that their bottom sides are the correct color. In the worstcase scenario, no corner cube has a bottom side with the correct color. In this case, do the following move:
R B R+ B

You only have to do this once. Now there is at least ONE corner cube is finished, with the right color on the bottom side.
Rotate the entire 
Repeat the move: 
...until both front  
R B R+ B 

________________________ 
What the move does: 
You may have to repeat the sequence twice to finish the front bottom corners.
Rotate the entire 
Repeat the move: 
...until all four  
R B R+ B 

________________________ 
What the move does: 
You may have to repeat the sequence twice to finish all four corners.
It is now time to solve the Bottom Edges.