Just remove them, clean them with a wire brush, do the inside and outside of the battery terminal and connectors with a cleaning brush (specifically made for cleaning terminals and cables, can be bought at any auto parts store under $10). Google "Clean battery cables" for easy instructions. You can even use a toothbrush and baking soda.
Then make sure you clean tighten the cables that go to the relay box. There is a small plastic cover that protects the bolt and cables. Make SURE IT"S TIGHT....this is the most common place for the problem.
After that then check and clean the starter end and ground end.
The start it up and check it out.....should be fixed.