Well, I know the title is a bit vague, and this would probably fit better in the creators' forum, but I couldn't find a good place to put this in there, so here it goes...

With my fresh copy of the legacy collection in tow, a creative spark has awoken inside me. Now, I like freetime quite a lot, but Desiderata Valley is a bit... lackluster in terms of storylines and lore, especially compared to the base game neighborhoods. Where I want to come in is by making a version with a bunch of storylines and drama added in- and a lot of it is based on some snippets of lore from the original. Now, I remember that years ago, I tried to something similar with Veronaville, but I basically ended up just asking everyone else to do all the work. This is not going to be the case this time: I'm willing to take my time to learn what needs to be done. Using CroconawSims's Desiderata Valley with townies and npcs as a base, I plan on also...

-Restoring the ancestral sims that Tarlia deleted
-Changing memories, relationships, and starting wants to fit the new storylines
-Renovating some of the lots (holy crap, the houses in Desiderata Valley are BAD. Unfortunately, I'm not much better myself, so this might take a while)
-Various other fixes and tweaks (aka why doesn't Natasha Una have a pottery talent badge???)

To make a long story short, I'd like you guys to help me a bit by pointing me in the direction of some tutorials that might help me with what I'm trying to do. I tried looking up guides for making neighborhoods, but most of them are for creating new ones from scratch...

You will probably find the information you need in the tutorials for making a neighbourhood from scratch. It's just that rather than from scratch, you already have some of the things you want to use.

Part of the learning process is also how to figure out what indeed even needs to be done - so it would be a good idea to go through the process of creating a neighbourhood from scratch, even if it's a crappy one with one house and two sims called Mr. and Mrs. Tester.

I would look up how to extract/clone sims using SimPE.

And you may find a use for the HoodReplace program.

But yeah, this is not really a beginners' project. And while I wouldn't let that put you off, it does also mean that you likely need to break it down into a series of mini-projects, and teach yourself each part, if you don't want to ask others to do the work for you.