Folders to backup before installing: texture/terrain and texture/water 
Go to your client's folder, texture > terrain. You will find a lot of images there which can be edited with any designing tool. I highly recommend using something like photoshop, gimp or similar, as integrated tools like paint might not give you the best results. You can check out
This Guide to see more about such editing tools.
Remember it is important to keep the image size and format intact, as modifying the image size or format might break the game. Also, as you may notice by now, the floor works in 'Tiles' (little squares that repeat), so whenever you edit it, make sure to make every tile seamless (this meaning every tile has to merge with each other in harmony) otherwise you will get a squary, patchy mess. You can always find tutorials on how to do this by simply looking up 'how to make seamless tiles in photoshop' and similar searches. You can always find seamless images in google as well which will give you a good helping hand!
Creative Terrain Folder Instructions:
- Download the Creative Terrain Folder
- terrain folder contains all the base game files
- terrain2 folder contains a guide on where each file is used
- put all terrain2 files in your texture/terrain folder. replace all and open the game
- you should now see the name of each file printed on the ground
- you may now manually edit anything you’d like, using the terrain folder or using any of the already pre-loaded tweaks in the Downloads Section Below.