Well. Say for example the larger phrase is "a number decreased by eight equals twelve."
1. Assigning a variable to the unknown quanity, commonly X.
2. Find two verbal expressions for the same value.
ex. "a number decreased by eight" and "twelve."
3. Write out a mathematical expression for each verbal expression.
ex. x - 8 = 12
After that we would solve out for the answer to 8, which would be equal to 20. Hopefully that makes some sense to you. If needed I can explain some more!