If your USB drive is large enough and you want to put the entire DVD on it, you can use isohybrid slackware-$VERSION.iso. Then dd the iso image to your flash drive.
If you have the files on a local mirror, another drive, or want to try the ftp install, use the small usb image under usb-and-pxe-installers directory. Read the README files. Note - it's best if you have a hard wired connection available.