This is what works for me:
1. Make sure your device is rooted and that game guardian has super users permissions (I use an app called Superuser to let me know anytime anything is requesting root access)
2. Configure your temp path through the Game Guardian App
3. Go into the settings and configure the game guardian icon to be in the top right
4. Load Simpsons Tapped Out
5. Open game guardian through the icon which sits on top of Simpsons Tapped Out
6. Search for the mount of XP current have.
7. Collect some XP from a building or quest that has completed
8. Search for the new amount of XP you have (without clearing first, you want to search within your search results)
9. Continue increasing your XP and searching for the new value until you see 4 exact matches, select those in the Game Guardian screen and change the value to match the amount of XP you need for a level.
10. Collect something worth 1 XP (Homers house is worth 1 and refills quickly)
11. At maximum level on the game when you gain another level you get to pick a reward box. This can be 1-3 doughnuts and you can pay to open the other boxes until you definitely get 3.
12. Repeat this process until you have 6 doughnuts (2 levels).
13. Now clear your search in Game Guardian and search for the amount of doughnuts you have (6).
14. Spend 1 doughnut on a white picket fence premium item.
15. Search for 5 within the previous search results and continue this until you have 4 values being returned. This usually happens within 3 spends of 1 doughnut.
16. Now you have the doughnut value which you can edit to be whatever you want.
NOTE: While changing the value of your XP or your Money is saved when you quit the game your doughnut value is reset the next time you load the game so make sure you do everything you want to do before you exit the game or you will need to repeat the entire process. If you want to be a bit more organised you can buy mystery boxes with doughnuts before you exit. There is a small chance that inside the box will be another box. Do this until you have about 30 unopened mystery boxes in your inventory. The next time you start the game open all those boxes until you get one which has a doughnut reward in it then you just repeat the steps from above starting at step 13. This does require a bit of luck with mystery boxes as it is random what reward it gives you.
Hope that helps.
Thanks to the developers of Game Guardian. Made playing Simpsons Tapped Out much more fun.
You can of course get the modded apk and install that instead which makes getting unlimited resources much easier ()
