Hey! Im GLACΞ, and I was just wandering, how do people make custom sprites and stuff like that? I would like to make a custom titlescreen, and a texture to a new powerup I made thanks!

Hello. Welcome. You can find some tutorials here:

http://nsmbhd.net/forum/22-nsmb-hacking-tutorials/

Theres one on how to edit a title screen. And much more IIRC, like editing the .ncl files and stuff.
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I made a world map tutorial here:

http://nsmbhd.net/thread/1857-how-to-edit-world-map-textures-tutorial/

Please note that this does not only apply to world maps; it applies to enemies, objects, jyotyu texture files, and more, too. Just open the folder "enemy" for all textures for the spike balls, Goombas, Peach, Koopas, Wigglers and other enemies/obstacles/NPCs. I'm not too sure, but I think Mario is in the enemy folder too. (Correct me if I'm wrong.)

Open the .nsbmd files for the enemies (Their names are written in Japanese, so if you don't understand what an enemy is, go over to Google and type "<japanname> mario" and replace <japanname> with the name of the nsbmd file (not counting .nsbmd).

Thanks