Firstly, You probably don't need to do this. I don't know a lot about it, but I know theres a way easier way, involving some software or something and there's a guide on that. Unless you want full control of the files of each individual mod, and/or Absolutely no ties between unstable and your main game, You do not need to do it this way.

The first thing is that Be aware that you may experience bugs or crashes on unstable that don't happen in the main game As far as I'm aware they're the same version and Unstable definitely isn't updated anymore so I don't think its actually much of a problem, BUT, you may want to make sure you know how to read the Starbound.log File to better understand what is causing problems and potentially what you can do to fix it.

The mods on unstable will not update. If a workshop Mod receives an update or change, You are going to have to repeat the process for that mod and replace the old .PAK file manually to "Update" the mod. This is the major downside of doing things this way.

Also, reminder to be respectful if you plan to do this. Don't just steal crap, do/don't modify it, and claim its yours, Mainly if you're going to upload this to any sort of site, or the workshop yourself. Ask first, and give proper credit.

Personally, I treat Unstable like a second, Independent copy for various purposes. Such as;

1. Using non-workshop mods, from places like modding sites.
I do this because I like to preserve Multiplayer with my main game as much as possible, and running any non-workshop mods That I like separately makes that a lot easier, cause I know they're things I don't wanna labor whoever I want to play with with installing, but I also don't wanna have to move them away, which would probably also for bigger mods mean messing with save data too, EVERY time I wanna do Multiplayer, so there's that.

2. Running Workshop mods I don't want affecting my main game.
So there's of course some workshop mods that I maybe kinda like, or impact save data too much for
me to want to bother using them, especially if I don't wanna use them all the time. Prime example, for me, is Frackin' Universe. I like some of the things It offers, but I would never give up my main save
data for it and I don't want its massive gameplay changes all the time, so I can run it independently
on my Unstable copy and not worry, as well as not bother with just moving save data and such around just for when I do or dont wanna run such mods.

3. Making personal tweaks or testing some mod handiwork of my own
I often make Personal-use tweaks to parts of mods, such as spritework changes, or compatibality patches for two mods to work together. Again, reminder to read the disclaimer if you're doing it for this reason.

Those are my main reasons I use Unstable this way.

I will now go through some file locations, techniques, etc. that you will need to know first. If you have
basic knowledge of where Starbound keeps its files, how to use the "mods" folder, How to find
the workshop .pak's, etc. then you can skip the first part of this part.

1. File types.
So first and by far most important thing to know, is where SB and SBU Keep their mods and how they're stored.

Mods are (typically) Stored in a (modified) ".pak" Format. Its not important what it is, just know the .pak's of mods are Uniquely tailored for Starbound, and as an extra tip, if for any reason you need to open or look at the insides of them, You'll need to use the Starbound Asset packer and Asset unpacker. There's very good separate guides by other smarter more qualified people then me, so Look up those if you're interested. Mods can also run in folder form, but this is vastly less efficient.

To run an external (as in, not automatically installed by the Steam workshop, which of course, our mods will not technically be.) Mods, you need to place the folders / .Pak's In a specific folder of unstable for them to work.

2. How to do it.

1. Start by going to Starbound Unstable in your Steam Library. There, In the list of games, Right click it, go down to "Manage", and Hit "Browse Local Files". This will take you to where Unstable is installed, Either note this path or just keep it open in A separate Window (or Tab if you have those Fancy things thanks to Windows 11), because its very important. If you open the "Mods" folder here, this is exactly where we're going to be placing our folders and .Pak's (I'm just going over How to move workshop mods, but just know as a tip, this is where you put Non-workshop mods as well, And regular Starbound has the same thing should you need it for some reason.)

2. Now that you have that, Here's where i'm going to tell you to open your browser. I know it seems weird, but follow me here. Open steam in your browser, and go to the Workshop. Then search your way to Starbound, for *It's* Workshop. (I'd reccomend you be logged in, so you can tell/go to the list of what mods you have installed, but its not required if you just already know the exact name of the mods you want or whatever.)

3. Here's where you'll need another File location, this ones not as easy. I feel like most people (especially those who are reading this), know where to find steams files.) But incase you don't, repeat the first part of step 1 to open where Starbound is installed, and at the bar at the approximate top of file explorer that shows a path (Which is like C:/Folder1/Folder2), Just go up and click "Steamapps" to take you there. From there, you're gonna wanna go to Workshop, then Content, then 211820 (this Number Is Starbounds Game ID Not important, but it is a fun fact!)

4. Now you're gonna see a bunch of folders with their names as scary long strings of numbers (or just a few depending on how many mods you have installed)

5. Now go back to that in-browser workshop Page, and Look up the Mod(s) You want to move's pages. If you look in the URL (That link in the search bar up there), there should be a string of numbers. That is the mods ID.

6. Now Match that Number in the URL with those folders I had you open, Once you find the Folder with the same number as the Mods URL, Open it. Inside there is a .pak file (Typically named "Contents.pak") COPY (It is VERY important you COPY the file, and NOT move it. Otherwise, the main game wont have the mod anymore or something...Probably. IDK i've never made that mistake) the file, and paste it in the "Mods" folder For Starbound Unstable.

And Voila! The mod should now work when you run Unstable, no further change needed. Though personally I Rename the file (And you will very very likely have to if you're installing multiple mods). Now, just do this for every mod you want to use in unstable and you're pretty much good to go!