Step one: take almost everything from one side of the basement and put it on the other side. Cram everything in as much as possible — make it look like a complete wreck.
Step two: clean and paint the now-clean side of the basement with waterproofing paint to reduce eliminate the risk of future flooding. (This should be done in conjunction with a complete renovation of the gutters’ drainage system.)
Step three: move everything else out of the basement storage room into only other room in the basement. Pack it all as ridiculously tightly as possible.
Step five (the previous one counts as two steps): clean the other half of the basement storage room floor
Step six: dread reversing all the steps.