I find myself needing to do this all the time and have to go search for a good artcile to remember all the commands. I often go back to this article to help me remember how to do it. For the sake of brevity I am just writing down the commands needed. Please read over the full article to understand what you are doing.

sudo fallocate -l 4G /swapfile
sudo chmod 600 /swapfile
sudo mkswap /swapfile
sudo swapon /swapfile

Now that we have enabled swap we need to make sure it persist when we reboot. To do this edit the /etc/fstab file and add the following line:

/swapfile   none    swap    sw    0   0