If you downloaded Saft Installer, run the installer and follow the instructions to install it.
If you get your copy of Saft in a disk image, then follow the instructions here:

1) Delete Saft.bundle in /Library/Application Support/SIMBL/Plugins if any
2) Move Saft in /Library/InputManagers to Trash if any
3) Copy Saft in the disk image to /Library/InputManagers and (re)start Safari

For Saft older than version 6.5.0:

If you do not have SIMBL (http://culater.net/osd/SIMBL/SIMBL.html) installed, go to SIMBL folder of Saft disk image, copy it to disk then install it. If you have SIMBL installed but it is a version older than 0.2b, you may have to install Saft inside SIMBL into the folder "plugins". So check the document, or upgrade to the latest version of SIMBL.

Delete Saft.bundle in /Library/Application Support/SIMBL/Plugins if any
Copy Saft.bundle in the disk image to /Library/Application Support/SIMBL/Plugins, then (re)start Safari


Removal
Run Saft Installer and choose uninstall, or Move Saft in /Library/InputManagers to Trash.