When you see a fraction, you see a numerator and a denominator. To find the common factor, see whichever number among these is least: The numerator, the denominator or the difference between numerator and denominator.
As well as a printed blank hundred square, another way to visualise decimals is to make a decimal stick. Get a cane or long stick, place with tape a number 1 to the right hand end, then a 0 to the left. Ask them now to place 0.5 in the middle, then together work out where all the other decimal points should go.