Welcome to the forums
It's nice that we are still getting new members after all this time, especially seasoned Lemmings fans. If any other Guests who haven't signed up are thinking of signing up, I advise you to go ahead and do it, we're all a nice bunch on here and don't bite! I find that level packs from newer members tend to be just as good as those from the veteran level pack creators, people also tend to prefer level packs with easier difficulty levels so I wish you the best of luck with creating your level pack.
It's nice that you have already played NeoLemmix custom content before signing up, I feel that playing packs created by other users is the best way to learn and make your own levels even better. You can even open other people's levels up in the NeoLemmix editor if you want to learn how to use the editor even better.
(I even have some tutorials on my Youtube channel on the NeoLemmix tools if you want to learn even more on using the tools, they've been helpful to other people I hear)
You can also post a development topic for your in progress level pack in the Level packs forum subforum "In Development" once you have a name for your level pack project. It can just be a working title if you haven't got a final level pack name just yet
Most of all, it's entirely up to you how you make your level pack. While others might give feedback on your levels after solving them on how to make them better, if you are happy with your pack, then that is the best thing of all