I had a leak under my kitchen sink which has caused some water damage to the bottom of the cabinet. Structurally it is fine, but I wanted to make the bottom look better. What is the best way to repair this (I have already repaired the leak)? I have purchased a thin sheet of wood to place on top of the original cabinet bottom, but I'm not sure how to get it to fit with all the plumbing coming in from the floor. Any suggestions?

Get the Dimensions of the piece you need to cut to fit the bottom of the cabinet. One way to do this is to get a piece of black felt paper, one big enough to cover the bottom of the cabinet, it does not have to fit perfically just so it is within a 1/2 inch of the walls of the cabinet. Tack it down with some thumb tacks so it can not move around, the hole around the pipe don't have to be exact. After the have to felt in place get a pice of wood that is 1 1/2 inch wide or something close. Place the strip of wood against one of the walls of the cabinet and then scribe a line beside the strip, do this on all sides of the cabinet, this will give you a border around the felt paper. Place the felt on you piece that you are going to use to cover the botton. Place the strip of wood on the same marks that you made on the felt and then mark the wood on the other side of the strip. This should give you an exact fit of the bottom.
Now about the pipes, measure from the back and from one side the distance to each pipe, mark each location on yoiur piece of wood. The use a square and mark a line from the back to each side of each pipe. Then use a drill bit just one size larger than the pipe you are trying to fit around. Do this for each pipe, I realize that you may not have a bit for the drain so saw the hole for it as needed. After you have the holes then use a saw and say the straight lines you made from the back of the cabinet. Save these pieces so that after you get the bord in place you can place the pieces behind each pipe... Let me know how this works for you.

It does not explain how to get the old water damaged material out from under the sink. It appears to go under the sidewalls.