Don't go by that link fancypiper posted, it only explains how to use FTP, not setup a server.
Slackware comes with two preinstalled FTP servers, wu-ftp and proftpd. I would suggest using proftpd as it has a better track performance than wu-ftp. You might want to look into something like sftp if your wanting a secure ftp server..
Basically the first step is to edit your /etc/rc.d/rc.inet2 file and make sure the inetd is uncommented to start at boot time:
Will look something like this:
# Start the inetd server:
if [ -x /usr/sbin/inetd ]; then
echo "Starting Internet super-server daemon: /usr/sbin/inetd"
echo "WARNING: /usr/sbin/inetd not found."
Now you'll need to make sure proftpd is uncommented in your /etc/inetd.conf file like this:
# Washington University FTP Daemon:
#ftp stream tcp nowait root /usr/sbin/tcpd wu.ftpd -l -i -a
# ProFTPD FTP Daemon:
ftp stream tcp nowait root /usr/sbin/tcpd proftpd
#telnet stream tcp nowait root /usr/sbin/tcpd in.telnetd
In most cases proftpd is already there by default along with wu-ftp which is the Washington University FTP Daemon. Just uncomment the proftpd line like above... uncommenting by removing the # mark in front of it. A # in front of config files will be ignored if you didn't already know.
If your wanting anonymous FTP server make sure you edit your /etc/ftpusers file and uncomment ftp in that file which the default ftp diectory will be /home/ftp. You can change the default home directory for user ftp in /etc/passwd to any other directory if you want to change it to another location. Regular users you might have setup that have a login to your server, their default will be their own home directory as well.. /home/username
Now if you want to have users with their own ftp directory to their home directory, but not have a shell access just edit the /etc/passwd file and make their default shell something like /bin/false
Then edit the /etc/shells file and add /bin/false to the list of shells. This will not enable them to login to the server by other means like ssh or telnet, or even at the machine itself.. only basically giving them FTP access to upload or download files on the server.
Hope this helps in setting up your FTP server on Slackware.