In this step we'll complete the centers of the middle slice while treating yellow, the opposite of our color from step 1, as if it were any color. By that I mean we're going to put all the center pieces of the side colors onto their correct sides and ignore any yellow pieces.
To do this, rotate the cube so that the white side is on the right and perform U2L2 to put the UR edge-corner pair into the DL slot freeing up the U layer.
Then, use the U layer and middle slices l, M, r to solve one center at a time. The first two centers should be pretty easy to solve just by taking one center piece at a time and moving it onto its corresponding side inserting them as bars once necessary. The last two centers are a bit more diffucult but you should be able to complete them using intuitiion.
When you're done with the centers, undo the setup moves with L2U2 and move on to step 3!
If you're really stuck on the last two centers, here's a simple algorithm you can use to swap any two center pieces or center piece pairs.
You can adapt that algorithm to any position of the pieces to be swapped as long as a slice move will but them in their correct spot. The way it works is you first push the center piece from the F side to the U side, then do U or U' to move it out of the slice it came from and then push the slice the piece lands in up as well. Then just undo the slices you did in the same order, not in reverse. Add U or U' in between slices to preserve the rest of the U layer.