Method A: Use Startup Applications GUI

    Open: Startup Applications (search in Activities or run gnome-session-properties)

    Click Add

    Fill in:

        Name: Posture Reminder

        Command:

        /home/your_username/posture_reminder.sh

        Comment: (optional) Reminds me to sit straight every 5 minutes

    Click Save

Now it'll run automatically every time you log in.