This is going to show how to edit the backround in new super mario bros, consider the following:

1.Open up the newest version of new super mario bros editor

2.Select a nsmb rom

3.Click the BACKROUND button

4.Choose the backround you want to edit

5.You can mix up the pieces of the backround just by clicking on them

Tip:You can import your own backrounds depending on the size of it.

I hoped this helped

You're missing things there
For example, imported backgrounds need to be 512x512 pixel PNGs. And they don't have to be too complex because the editor needs to reduce the number of different 8x8 tiles

See a lots of creative DS Hacking here
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, never knew. I looked at the tutorial thread and there was no backround tutorial