The Secret Saw can be found in the chest seen below, just before chapter 3.

To reach this area, go through Chapter 4 like normal, however instead of going through the gate to transform back into a golf ball after the saw blade levels, head back the way you came through the entrance you took when beginning this section.

In the case that you've already found the "Secret Saw" before this achievement was introduced into the game, you will need to do one of two things:

1. Replay through the game again until chapter 3/4 to rediscover it again unfortunately...
   
2. Load a save from just before opening the chest (MUCH faster)

The game saves can be found at the below location (make sure to back up your own save in a different space before deleting / replacing it).

• %USERPROFILE%/AppData/LocalLow/Triband/WHAT THE GOLF_

User tipiseb points out that past saves can be found in this save location in the Backup folder, and with a little bit of trial and error, the game can be loaded back to just before collecting the saw. Alternatively, I have a backed up save from moments before opening the chest, you just need to reassemble the google drive link below to access it.
https:// drive.google.com/ drive/ folders/ 1TN6INIis0ILpO_MtakiKh_TdmMcZymMj

Steam User 逸 has kindly reproduced tipiseb's method as a guide in Chinese for those interested:
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=3223156460

And User Nebula notes that you can create a new save file via the Settings menu when in the hub world (Settings -> Save Slot). Here you can create a new save by clicking on one of the other golf flags, or reset your current save by clicking on the currently selected flag. Much easier than going into LocalLow and deleting files.