Take a look at /usr/doc/ntp-4.2.6p3/ntp.SlackBuild
Pick a directory of your choosing, mkdir ntp, then mkdir ntp/src
Copy the SlackBuild to there. Edit for the very latest version. See if the script builds the package, including grabbing the correct source for itself.
If you have it and would like to submit, post to "Packager Training and Help". This will remind me to write up some docs I've been promising the repo maintainers for a while now.
Note: For anyone doing this with other packages, you should check also for patches, and you'd need that section of the source repo (vabs) tree for that. I've checked on this one already, and there are currently no patches for ntp to date.