It is easy to do. As you have already partitioned your disk, the most recommendable is to create a partition file. You must first create a file of desired length with "dd", then use mkswap to prepare it and after just enable swap. It is important the file be created with "dd" because it has to be a contiguous, not an sparse file. So, the commands to do it are
dd if=/dev/zero of="/path/to/your/swap-file" bs=1k count="your-need-in-MB"x1kx1k
To swapon it on the start up, you have to add the following line in "/etc/fstab"
"/path/to/your/swap-file" swap swap defaults 0 0
"/path/to/your/swap-file" you must replace with your directory and file
"your-need-in MB" means the number of mega bytes you want. Observe we are multiplying bye 1kB two times. So, if you want 256MB, the command will be 256x1kx1k. If you want 1GB, you can put 1kx1kx1k.
PS. you can get more information on manual page for mkswap. Linux can work with more than one swap area.