While this really is simple. I'll over-explain just in case. TL:DR at bottom.

Before we start, Windows has a very stupid "feature" enabled by default. Go into your folder options and turn off "Hide known extensions". What does that option do?
gets displayed as
Nice way to disguise a virus right there. If will also get in your way modding too. Turn that "feature" off and never turn it back on.

• Download Notepad++ to do your editing in. Or your preferred advanced text editor. Anything better than Notepad.
  
• Here is a tutorial if you need it, to cover packing/unpacking mods
  https://community.playstarbound.com/threads/how-to-successfully-pack-and-unpack-pak-files.66649/
  Since you may have to unpack the race mods. Some mods downloaded from the forums may already be loose files though, so they may not need unpacked.
  
• If obtained from the workshop, you definitely need to unpack it but first you need to find it. The exact path will depend on where you installed it. Workshop files would be in
  /Steam/steamapps/workshop/content/211820/<id>/contents.pak
  <id> you swap for the id found in the URL for the mod's page. Case in point this mod's URL is
  
  steamcommunity . com/sharedfiles/filedetails/?id=1243244711
  So for this mod the ID is 1243244711 - thus this mod can be found at
  
  /Steam/steamapps/workshop/content/211820/1243244711/contents.pak
  The same is true for any mod on the Workshop, from any game that supports it.
  With that, you can always find where specific mods have downloaded, and thus unpack mods from the workshop. If you are curious, 211820 is Starbound's gameID, which you can find from the store page URL.

You need to create a folder within /starbound/mods/ with any name you like, but the name should describe the contents such as "FelinRace_VeprShip" (for example). This folder should have these two files in it.

• _metadata
  
• universe_server.config.patch

To make each file within the folder - right click, make a new text document, then rename it.
Open them in notepad++
As for the contents, you can copy/paste the text here, then edit it to match what you are doing, save. I'll give you an example that would put the Felin race into this ship.

_metadata
Swap out "author", "description", "name", and "friendlyName" with your own info.

"includes" - this causes your patch to load after the mods you list there. Swap out "Felins" with the mod name of the race you intend to support. Get that info from that mod's own _metadata file.

"requires" - this lists the mods which must be present, or Starbound won't load (and there will be a log entry to explain the missing requirement). Like "includes" it will cause your patch to load after those mods.

"bk3k_blocks_and_objects" is where all the assets are for the mod, hence it is required.
"Vepr Ship(revived)" is of course this mod. While it doesn't have assets, it does have recipes and directs Starbound to load a script that fixes the crew maximums per upgrade level. So you just leave "requires" as is.

universe_server.config.patch
That of course puts the Felin in the Vepr ship. Swap "felin" for whatever race, and it'll work for them. You can get their name from that race's own universe_server.config.patch file, although in many cases you can probably guess it without.

By all means go out there and patch your favorite races to use this ship. And publish your patches if you like so others can use it.

Well other guides should exist, but this isn't hard. You can directly upload the files on the official SB forums for non-Steam users easily enough just go to that site and Click the "Add mod" button relatively on the top right.

As for Steam - when opening Starbound you get that menu option on bottom "Launch Mod Uploader Tool". It opens a simple program - you browse to your folder, Make sure your description data says what you want it to say etc, and hit "Upload to Steam!" As long as you don't get an error, it is done and you can close the program.

There is one more step so people can SEE your mod. Open Starbound's workshop page.
There is a link on the semi-top-right "Your files". Click on that.
"Files you've posted". Click on that.
Click on your mod from there, and you'll see your workshop page. You've seen others, but this one is YOUR page. There are some controls you haven't seen before - since this is your page.
You'll see "owner controls" on the right. Change visibility to "public" and in a few minutes everyone will be able to see your mod. Granted if you really want, you can set it to "friends only" but you might as well let everyone use it. Your choice.

Make _metadata and universe_server.config.patch files
Swap "author", "description", "name", "friendlyName", and mod names in _metadata.
Swap race in universe_server.config.patch.
Done.