[GTALUG] How to run script at bootup in Debian

Hi all, I need to run a custom script at the later stage of bootup in Debian 9.5 (board is BeagleBone Black).  I seem to remember "rc.local" long ago, but it's gone in the latest Ubuntu and Debian.  Online search says     (1) simply create "/etc/rc.local", and systemd will automatically pick it up.     (2) use crontab, "@reboot ..." Which is "proper" way in Debian world?--William