Shut off power and gas
The first thing that you will want to do if your basement is flooded is to turn off the power and gas to the area. This can usually be done from the main power box or the gas valve. Be careful that the area is completely dry before trying to turn off the electricity.
It is very important to remove all of the water as soon as possible. For this, you should use a high power water pump. Once you get out most of the water, use the wet / dry vacuum to take out the remaining puddles. You can also use floor wipers to help collect the water as you use the vacuum.
Air dry basement
Once majority of the water is removed you can run some extension cables to plug some fans in to help air out the basement. If you have any windows in the basement then open those as well.
Pick up everything
Remove everything that was underwater in your basement. This usually includes furniture, rugs and carpets. Drag everything outside and let it dry out in the sun if possible. Take out any electronics and also let them sit somewhere to properly dry.
Take some bleach and hot water mixture to disinfect the basement floor. The purpose is to remove any bacteria that might have been a part of the flood water. It is very important to throw out any items that might have mold or mildew growing on it. Take your time to disinfect the basement as this will help reduce the nasty odors as well.
Check wiring and plumbing
It is a good idea to check all the basement wiring and plumbing to make sure it was not damaged from the flooding. Check each socket to make sure everything is working properly. You may want to replace some of the wiring if it was submerged underwater for a long time.