6/12=60/120=600/1200= etc. because we multiplied top and bottom by the same number.
Let's take 60/120, we can write this as 60/(12×10)=5/10 by dividing top and bottom by 12. 5/10 means 5 tenths. One tenth is written 0.1, so 5 tenths =5×0.1=0.5. That means that 6/12 = 1/2 = 0.5.
So the long division is worked out by putting in a decimal point after 6:
0.5
12 ) 6.0
12 doesn't go into 6 but it goes into 60 5 times. Therefore we put 0 over the 6, then we have the decimal point followed by 5 to show that 12 goes into 6.0 0.5 times.