First, create a file that will be used as swap:

fallocate -l 2G /swapfile

If the fallocate utility is not present on your system, or you get an error message saying fallocate failed: Operation not supported, use the following command to create the swap file:

dd if=/dev/zero of=/swapfile bs=1024 count=2097152 (4194304)

Set the file permissions to 600 to prevent regular users to write and read the file:

chmod 600 /swapfile

Create a Linux swap area on the file:

mkswap /swapfile

Activate the swap file by running the following command:

swapon /swapfile

To make the change permanent open the /etc/fstab file:

nano /etc/fstab

/swapfile swap swap defaults 0 0

Verify that the swap is active by using either the swapon or the free command, as shown below:

swapon --show

Source: How to Add Swap Space