git clone 'https://github.com/shenso/xfce4-xsecurelock' $HOME/.local/src/xfce4-xsecurelock
chmod +x $HOME/.local/src/xfce4-xsecurelock/install.sh
sudo $HOME/.local/src/xfce4-xsecurelock/install.sh

You may need to log out of your current session for the new command setting to be used. If running the command xflock4 does not lock your screen, then double-check that the lock command has been set to xfce4-xsecurelock in the settings editor.