How to work out Raito into fractions

I am assuming you mean Ratio into Fraction.

It is really simple, actually. Just put one number on top of another (divide would be a better word).

I have a ratio A:B, which is 1:2.

I can put 1 on top of 2 to create the fraction 1/2 to say that A is 1/2 of B.

A=1/2B

I can also put 2 on top of 1 to create the fraction 2/1 to say that B is 2/1 of A, or 2 times of A.

B=2A