Sorry if this guide already exists somewhere on the domain, but I did not see it so I decided to make a tutorial for people who don't know how to do it.
I didn't check any site. Just found out by myself.
Remember that this is for editing world map textures, not editing whole world maps with new models and paths
Here it is! It is not that hard.

1. Open up NSMBe and choose any ROM file.

2. Click the "ROM File Browser" tab.

3. Open up the FILESYSTEM, by clicking the "+" behind it.

4. Open up "root".

5. After opening up, you will see a lot of folders. Find "map" and open it up.

6. You will see a lot of files with the extensions ".nsbca", ".nsbtp" and more. Double-click "w1.nsbmd". Make sure that if you are editing any other world, it should have "w" and the number of the world like if you are editing World 2, find "w2.nsbmd" and if World 3, find "w3.nsbmd" etc. In this instance we are editing World 1 so we choose "w1.nsbmd".

7. There will be a window. Choose the texture you want to edit. All textures are listed in "Bitmaps" and the palettes are listed in "Palettes". All textures use their own palette.

8. Once confirmed what texture you want to edit, click "Export this bitmap". In the dialog box, give the file a name
and choose what folder you want to save the .png file in. After confirmed, click "Save".

9. Open up your favourite image editing program (In this instance I will be using Macromedia Flash. I don't recommend Paint for texture-editing). Recolor it, make new textures or do whatever you want. Doesn't matter

10. Export the file. Make sure the image editing program has a "Export to PNG" feature. Since I am using Flash, I go to File > Export > Export Image and choose PNG extension, give my new texture a name and click "Save".

11. After done, switch to the NSMBe texture editor window. Make sure you selected the same texture as you edited or the map in-game could go wrong!

12. Click "Import this bitmap and recreate palette". Choose your newly made PNG texture image, and click "Open". Yay! You don't need the save button. Close the window and open up the NSMB ROM with an emulator or a real DS.
When you go to the world map you will see the texture(s)!

You can edit as many textures as you want at the same time.

BTW, Here is my edited world map:

Can you add a 'tutorial'-thing (How do you say that? ) at this thread?

EDIT: I'm sorry, it's already in the title.
Ignore this.

No problem.
By the way it is called a "Tag".

@Dirbaio
It's true.

I probably would have needed a tutorial like this at some point.

how do I place the trees in the world 3 without replacing the existing ones? I'll explain: skawo is doing a rom hack and near the house of boo put of green trees but only at that point. how do I do the same ?

Well. This is however offtopic. Because your question doesn' t concern with texture. It is model editing.

But: ahh small tutorial.
Open MKDScm and your ROM.
Find in map folder w1_tree.nsbmd
Open that file. Export model as .OBJ.
Open Sketchup, import that file and edit it.
Export the OBJ. Then on MKDScm click on New, then in NSMBD and select your edited .OBJ.
Select where do you want to save the NSMBD. Click OK. Replace w1_tree.nsbmd with your .NSBMD.

I did not mean just this also because I know to do , what I mean , and that skawo 4 in the world has put the boo suspended in the air I would like to figure out how to do and then 3 in the world where there is the level 3 -A has put as a pond do I do it ?

With the same method. You have to import the Boos in the .OBJ, in the right position. The same thing for the pond.

Besides sketchup Which program I can use?

Hey, i hava a problem when i want to edit the sky. If i do it, the Sky turns black and if i Import the old "picture", its still black. Can anyone help me?

I have a question, how do you edit the green part of the background

it's a texture like everything else.
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No, the green layer the castle is on, I don't think it is a texture (If it is, can you show me what the file is)

@SnakeBlock Hello! Do you know a tutorial for Goombatlas ? I searched a lot but I just found ray's post and I do'nt really understand how to export the world map.

Thank you

There's no goombatlas tutorial outside of what is in the official thread. You have to figure it out with trial and error.
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