Now that you have finished the entire Top Layer of the cube you are ready to start solving the Middle Layer.
The Middle Edges
When you finish this next step your cube will look like this:
Before we start, rotate the top layer until the center colors on all 4 sides match the top row. This will in effect lock the two top rows into position. We're getting closer. Your cube should look like this:
There are only 3 possible scenarios you will face when tackling this middle row. To start, simply look at the 4 edges on the Bottom Layer and find one piece that does not belong on the Bottom Layer. Once you have found one, simply rotate the Bottom Layer until you match one of these two possibilities as shown below, and follow the direction below the appropriate match:
The third possibility is that a piece is already in place, but backwards, as shown below. Unless the piece is on the bottom row you will not be able to get it properly in place, so simply put a bogus piece, from the bottom row, in its place - this will force the problem piece to the bottom row so that you can use one of the two methods above to put it back in place.
Repeat this process for each edge piece and you will have completed the middle layer.
When you have completely finished the Middle Layer,
you will be ready to SOLVE the Bottom Corners