Assuming you want to put it in '/', create an empty 1024MB file:
dd if=/dev/zero of=/swap bs=2048k count=512
Format it as a swap file:
sudo mkswap /swap
Add it to your running Linux system:
swapon /swap
Add /swap to fstab for automatic swap activation:
echo "/swap none swap sw 0 0" >>/etc/fstab
Verify that you've added the newly created swap to your Linux based operating system:
free -m