It depends :)
You want to turn the machine on and the next thing that you get a prompt from is a DM screen, or do you want to start it after you login on console automatically, or after you login on console to type lxdm,sddm,lightdm etc?
For example for sddm there are two s6 related pkgs (-s6serv and -s6rcserv I have yet to fully understand what each is appropriate for and what the difference is) but together with the sddm pkg and s6opts enable sddm will make sddm start up right away. For lxdm there is lxdm-s6serv .
If you don't enable it you can still start it from console manually.

Since I don't have other users loging in I have my wm specified in $home/user/.xinitrc and I login on console and go straight to my desktop, no DM.

start the corresponding service e.g.

# s6opts enable lxdm

obviously the service need to be installed on the system first e.g.

# pacman -S lxdm-s6serv

Refer to the wiki for futher information about s6opts : https://wiki.obarun.org/doku.php