The easiest way should be with the PackToolkit that come with the NL download. It should be present in your main NL directory.
This tool is no longer supported, and is certianly
not included with the NL download anymore - but
the last version of it can be downloaded standalone. You're free to try using it, but be aware that it's no longer being updated and does not support 100% of features. If you are using it, be
very pedantic with keeping backups of your work - it's had some errors that cause data loss in the past.
You could also peek inside the "levels" folder and see how the other packs are structured and manually create a new pack by creating a new folder inside the "levels" folder and mimic the other packs. Help for that: https://www.lemmingsforums.net/index.php?topic=4335.0
I've updated this topic with a new post, that takes a different approach - whereas the first post is more a list of "here's everything you can do", the second post is a step by step to go from "I've just got some random levels" to "Here's my pack, properly sorted into ranks etc".
https://www.lemmingsforums.net/index.php?topic=4335.msg77545#msg77545