First you need each side of the ratio to be in the same units.
The easiest way to do that is to convert 1 hour into minutes.
1 hour=60minutes
So your ratio is now 60:45
Now you need a common factor that goes into both 45 and 60.
15 is the greatest common factor so now divide each side by 15.
45÷15=3
60÷15=4
The ratio, in its simplified for is 4:3