Install the or AUR package.
slock to lock the screen. You can also provide an argument to be run after the screen has been locked:
$ slock cmd [arg ...]
To unlock the screen, just type your password.
Tips and tricks
Lock on suspend
Create the following service which turns off the monitor and locks the screen.
[Unit] Description=Lock X session using slock for user %i Before=sleep.target [Service] User=%i Environment=DISPLAY=:0 ExecStartPre=/usr/bin/xset dpms force suspend ExecStart=/usr/bin/slock [Install] WantedBy=sleep.target
firstname.lastname@example.org systemd unit for it to take effect for the username user.
Block VT switching and prevent killing X
slock recommends blocking VT switching so that the screen lock cannot be bypassed. For the same reason, slock recommends preventing users from killing the X server. See Xorg#Block TTY access and Xorg#Prevent a user from killing X.