If you're going to do it during install, run cfdisk
and create the partition, then format it and name it (all done in installation process).
If it's already installed, and you've got the room on an existing partition, you can either create a new directory on an existing (large partition or directory like /home) and store everything there. This is what I have done. My /home directory has 11GB available. So, I created a directory called /home/downloads.
as root of course.
Make sure that the directory is accessible by your user now by
ls -l /home/downloads
If it looks like rwxr--r-- then your user won't be able to write to or execute from it, so
chmod +w /home/downloads
will give root, user and world access to read and write and will look like this after ls -l command:
If you want to be able to excecute, you can chmod +x /home/downloads
or if you want to do it all in one fell swoop instead of one permission at a time:
chmod 777 /home/downloads
Personally, mine is set up as rwxrwxr--. I did:
chmod 774 /home/downloads
As for creating a partition on an already up-and-running linux system, someone else will have to help you out there. I'm sure there's a way, but I'm not 100% on how to do it. Hope this helps.