I can explain what I did if that helps,
I installed SDTD Editor Latest started it up selected my 7 days to die installed folder (after reinstalling a fresh copy) of the game, I then clicked Manual Update which installs the editors DLL into your steams game to enable the BBB code your going to use...
I then download Zombie editor which is a cool 2d style editor that allows you to resize & optimise your map after its saved... IE when you save the game saves from bedrock to sky so lots of blocks that arent needed...
Cant really help you with how the editor works except to say have a go, Top view is the one I use first to remove all the blocks from around the prefab, Side view I then use to remove excess air & dirt blocks leaving the prefab, I always leave 2 layers of AIR above the top most part of the prefab....
BBB code thats needed as in basic is,
bbb rp (sets you as admin)
bbb claim thenameofyourprefab
bbb exportsw thenameofyourprefab
This gives you the files needed for the zombie editor.... from there you place the edited files into the editor inside prefabs folder, you then click empty world & do the following...
bbb rp
bbb ee
then press F1
prefab load thenameofyourprefab
prefab simplify
prefab combine
prefab export
that gives you your .Mesh file to add to the other files...
Theres more tyo this but that is the basics of what i did.... watching Utube vids helps alot & the prefabing tut on the main pag of prefabs is awesome, I think Guppy also has vids which ive watch & helped too....