There are 2 lawn mowing jobs that need to be completed. They are located in the Vale. One is a park, and the other is a home. You can find them by looking for a green $ sign. This option is unlocked after the Vale is opened to you, and you have served at least one detention where lawn mowing was your punishment. In the park, there will be 3 levels. The first level, you must complete 70% to pass. You also have the option of going for the full 100% for double your payment. Then level 2 is the same park, but now 80% is the cutoff, then level 3 is 90%. The same goes for the home job as well. After you are done with all 3 levels of a location, you may continue to complete level 3 for extra money.
Use to accelerated the mower. Make sure to not run over the flowers or lawn ornaments (except gnomes), as it will cost you money.
For this achievement, there is a bit of a glitch. Sometimes, you only have to complete the 3 park levels for the achievement, other times you must do both. You are required to do both however for the 100% achievement.