Saft for Leopard:
1) Copy Saft.app to the applications folder.
2) Run Saft.app and choose to install Saft as an InputManager plugin if you have not done that with an eariler version.

Saft for Tiger:
Run "Install Saft" in the mounted Saft disk image.
In case you have problem with "Install Saft", follow the following instructions to install it.
1) Move Saft in /Library/InputManagers to Trash if any
2) Copy Saft in the disk image to /Library/InputManagers and (re)start Safari

Removal

Move Saft in /Library/InputManagers to Trash and restart Safari.
Finder may say that Saft is in use when trying to empty the Trash. Logout and login again (or restart mac) to solve this problem.