It goes like this

1. create 2 distinct numbers by isolating the last digit from the other. For example, 154 becomes 15 and 4.

2. double the number derived from the last digit. So, the four becomes 8.

3. subtract from the number derived from the preceeding digits. 15 - 8.

4. the resulting number is 7. Seven is divisible by 7, so we know 154 is divisible by 7.