Simplifying a ratio means writing it with the smallest whole numbers that still describe the same mix. The whole job is two moves: find the highest common factor, then divide both parts by it.
The Two Moves
One — find the HCF. The highest common factor is the biggest number that divides both parts exactly. For 12 : 18, both divide by 6, and nothing bigger does, so the HCF is 6.
Two — divide both parts by it. 12 ÷ 6 = 2 and 18 ÷ 6 = 3, so 12 : 18 = 2 : 3. You're finished when the two numbers share no factor bigger than 1.
A Worked One
Simplify 20 : 30. The HCF is 10. 20 ÷ 10 = 2, 30 ÷ 10 = 3, so the answer is 2 : 3. Notice that's the same simplest form as 12 : 18 — they were describing the same mix all along.
It's Just Like Cancelling Fractions
If you can simplify 12/18 to 2/3, you can simplify the ratio 12 : 18 to 2 : 3 — it's the exact same skill with a colon instead of a fraction bar. Reuse what you already know.