I'm using Paint (to resize the tiles 200%, copy+paste) and then I use PhotoFiltre 7 (to make the tiles transparent) lining them up in a Tilesheet template grid 256x19 999 px, which I know is the limit of RMXP.
Now I'm trying to do this just with 2 tiles of the nature pack eg. the grass flowers and 2 tree tiles (for mapping rows of trees)
and everything else from the building and urban pack, just the basic tiles needed to make a city.
The rest I will separate into another Tileset, therefore I'll look how it was done in Vanilla Essentials:
Bike Shop Interior, Boat, Caves, Department Store Interior.... etc.
I'll do it the same way, just with HGSS Tilesets (I'm not done yet)
thanks to this dude who shared his awesome work!