Encrypt the Folders
Ubuntu currently (November 10, 2018) has
CryFS as part of its packages so you can install it with apt.
sudo apt install cryfs
Create the Target and Source Folders
cryfs command will create the two folders and set them up for you. The syntax is
cryfs <target> <source>. The
target will contain the encrypted folders so it will go in the Dropbox folder, while the
source will hold the unencrypted files.
cryfs Dropbox/encrypted dropbox_source
This is the same command you would use on another computer to set-up the existing encrypted folder on your new computer. The
source folder can be named differently, but the
target folder and the password need to be the same one you used when you created it.
If you need to unmount it you can use fusermount.
fusermount -u dropbox_source
Since you are doing all this within your home directory you don't need root privileges (except to install
cryfs with apt).