Firstly, if you wish to create your custom ground, please make sure you have your graphics already.
You will need to add them to a spritesheet, if not done so already, go to your RotMG Source Folder (Client, not server) and under the 'src' folder look for: lofiObj3
You should find a .png file, open it up in a text editor, go to the very bottom where assumably the Ring of Decades is the last item in the spritesheet, go to the right of it and just simply import your 8x8 sprite.
== dat0.xml Part ==
Open your server source and search for 'dat0.xml'
Inside, search 'gold sand' (as an example) and copy it from the beginning <Ground> to the end </Ground>
It should look like this:
Now, simply change the TYPE to something not in use, attempt to use a low TYPE number since it can cause lag if too high,
for example use 0x820
Change the ID to something different, such as: "Fur Block" (just name it whatever you want!)
Then, under textures, go to "lofiEnvironment3" and replace it to "lofiObj3" ..
if it is already lofiObj3, just skip this step.
Change index to: 0x565 (This will be the slot after ring of decades, increase the final digit for each sprite you add until reaching 9, then use alphabetical order starting with the letter
a and so fourth)
Then your set! Dat0.xml is complete. Move to next step.