How to clean a flooded basement

Access the damaged area
First of all, you must understand that trying to fix the problems caused by a flooded basement yourself, is not the safest of options.  If you only have a small amount of water in your basement, you will probably be safe removing it yourself.  But if you have a large amount of water in your basement, then it is likely accompanied by other major problems such as electrical issues, gas leaks, structural damage, and the such.  This is not minor task, and a professional should definitely be consulted.  There are several places you can find on the net who will remedy the problem for you with a simple phone call.

Turn off the electricity
Why?  Well, when was the last time you dropped a plugged in toaster into a kitchen sink full of water?  Everyone knows that mixing water with electricity is a deadly mix.  

Remove the water
If you only have a little bit of water you can simply get it up with a mop and a few towels
If the problem is much bigger then you will need to have access to a sump pump or wet vacuum.   With a sump pump you will need to make sure there is an available area for the water to be drained…as it wouldn’t make sense for you to drain the water, only to have it come right back into your basement.  If you decide to go with a wet vacuum be prepared to have to empty the water out of it each time it fills up.  As you can see, you CAN clean up your flooded basement by yourself, but it wont be a simple job.  Consulting a professional will be as simple as a phone call, and will relieve tons of stress.

Start in the proper place
Begin extracting the water where it is most likely to become damaged the greatest by the water.  If you cannot access that area, then try to start where the least water is and work your way to the larger pools of water.

Dry the area to avoid mold & mildew
Yes you must now dry the area ASAP.  Turn up the heat, and open the windows.  The heat and windows must be done together.  If you ONLY turn on the heat without opening the windows, it will only add humidity to the area, because the moisture will not be able to escape.  As a word of caution, be sure to only consult a professional if your heating system has been damaged by the water.
Turn on fans positioned so that they are blowing the air out of the room.  This will quicken the drying process and aid in preventing the dreaded growth of mold/mildew.
You can also use a dehumidifier.  The method for drying out your flooded basement will be somewhat different when using a dehumidifier meaning, in this case you will want to close all of the windows so that the dehumidifier can properly gather the moisture.  Keep watch on it, so you can empty it as it fills.  When one area seems to have no more water that can be extracted, place the dehumidifier in another area.

Clean up the “stuff” left behind
Yes after all of the water has been extracted there will be a mess left behind that could include mud, and etc.  You will want to clean all of this up and sanitize the entire basement.  In fact, sewage could even be in the mess left behind, so be sure to thoroughly sanitize.

Basement Finishes & Proper Furniture
A huge part of cleaning your flooded basement is having finishes and furniture that will be able to hold up to a possible flood, as well as be easily cleanable.  Ceramic tile is a perfect option.  As far as furniture goes, it would be helpful if the legs were long enough so that fabric was left untouched by the water.