There are tutorials for setting up LUKS encryption on secondary drives in elementary OS, but is there a simpler way to do this with the existing graphical interface?
The easiest way is to install
sudo apt install gnome-disk-utility
which you can run as
gnome-disks in the Terminal or as "Disks" from the Applications menu. Then simply select the drive and in the "Type" selector, choose the LUKS encryption option. It will prompt for a password (for the drive, not necessarily your login password) and you're good to go!
You'll be asked on your next reboot how you'd like to unlock/mount the drive in the future. If you'd like to compare the options, look here.