1. choose a location where you have lots of space (/usr comes to mind)
2. create a file the size you want the swap to be:
dd if=/dev/zero of=/usr/swap bs=1024 count=100000
(the above will create a file 100Mb in size, so adjust the count as yousee fit)
/*
Here may be wrong. Comment by baif.
*/
3. run the command: mkswap /usr/swap
4. run the command: swapon /usr/swap
5. check w/ free that you just added 100Mb of swap
6. if you want this to be permanent through reboots, you'll need to add the appropriate line to /etc/fstab