If someone can explain to me how to make a mod...
Mods require no packing whatsoever, but you will want an unpacked assets directory so you can copy the appropriate paths.
1. Starting with an empty directory, create a text file called <your mod name>.modinfo. Fill this with the following:
{
"name" : "<your mod name>",
"version" : "Beta v. Furious Koala",
"path" : "."
}
2. In the directory, create the subdirectories that trace the paths to the files you modified (in your case, biomes/surface/forest, etc.). Place your modded files in the appropriate locations.
3. Place the directory into Starbound/mods.
4. For uploading purposes, create a readme.txt and compress the directory into a .zip file.
If needed, you could download that mod I linked and follow it as an example.
The real problem with modding at this stage is that you'd have to update the mod with each new version of Starbound released, at a minimum changing the "version" line in the .modinfo file. You'd also need to check their .surfacebiome files each time to see if they changed anything, and modify those files if they did. Otherwise, your mod's users would unwittingly be updating their games but not their biomes. Basically, maintaining the mod takes work, and with the game still in active development, you'll end up updating it often. Alternatively, give people permission to modify your mod without your consent and hope someone else decides to take up maintaining it.
Not that I'm trying to discourage you or anything; modding's great, but there is a little responsibility that goes along with it.