This guide will assume that you already have Haxchi installed on your Wii U. If you haven't, please check this guide to find out how to get it.
Strap yourself in, this one is going to be a long one. Not hard, just long. This could easily take 2-3 hours.
If you're ready for it though, let's get started!
PART 1: GETTING THE GAME FILES
Make sure that you have at least 13.8 GB free on your SD card.
While it isn't nessecary, it's highly recommended that you go into the settings on your Wii U and either turn off Auto-Shutdown, or put it on a value much higher than 1 hour. 3 or 4 should be fine.
Start up Haxchi on your Wii U.
If you don't have it already (which you should), download SDcafiine through the Homebrew App Store.
If you don't have it already (which you might now), download Smash2SD through the Homebrew App Store.
Start SDcafiine. It should take you back to the main Wii U menu.
Start Mii Maker. It should take you back to the Homebrew menu.
Choose "Smash2SD" from the list of options in the Homebrew menu.
Make sure if you own a disc copy of Smash Bros for Wii U that it is inside the console.
You should be taken to a screen with 5 options. If your game is a disc copy, press A with the arrow over the "GAME FILES: DISC" option. If your game is a digital copy installed to system memory, press A with the arrow over the "GAME FILES: DIGITAL GAME INSTALLED ON SYSTEM" option. If your game is a digital copy installed to USB, press A with the arrow over the "GAME FILES: DIGITAL GAME INSTALLED ON USB" option.
You'll need to wait for quite a while. This process can take around an hour to complete.
Once it is done dumping the game files, dump the patch files. If your patch files are on the system, press A with the arrow over the "ON SYSTEM" option. If your patch files are on a USB Drive, press A with the arrow over the "ON USB" option.
You're going to have another fairly long wait. Not as long as the first, but fairly long nonetheless.
Once the patch files are done, exit out of the program / shut down your Wii U.
Alright! Assuming the program didn't randomly glitch out you should have all the files you need. You can't mod just yet though, there's a few more things you need to do.
PART 2: ORGANIZING / PREPARING YOUR GAME FILES
Put your SD card back into your PC, and make a folder on your PC, name it whatever you want. I'd recommend naming it "SmashDump", but you can name it whatever you want, it doesn't matter.
Go into the smash2sd folder on your SD card then enter the game folder. (So [SDCARDROOTLETTER]\smash2sd\game, and drag the code, content and meta folders inside of it into the folder you made on your PC.
Open the Smash2sd folder on your SD again, and this time drag in the code, content and meta from the update folder into the folder you made on your PC. Make sure you're dragging it into the white space of the folder. Basically, you should be in the part of the folder with the code, content and meta from the game folder you made, and then drag the update part into the white space of that folder.
Okay, now you're done preparing the files. Next you're going to have to download some programs. I'll explain all the programs and what we do as we go through them.
That file contains SmashCommand (nightly and normal), SmashForge, Sm4shExplorer, Sm4shTools, PACKMANAGER (with a custom script to make things easier), and FITC and FITD (also with a custom script to make things easier).
Smash Command (Nightly Or Normal) is your hitbox manager, SmashForge is your visualizer and .bin (param) editor, Sm4shExplorer is your file manager and the thing that compiles your mods. Sm4shTools shouldn't be needed, but figured I should put them in anyways. FITX can decompile your moves and let you edit them with a text editor, and PACKMANAGER can help you use moveswaps easily
Start Sm4shExplorer, click version 304 (assuming you have V 1.1.7, if you have 1.1.6 put 288)
Now you're set up! But let me give you some links that'll be useful for the next steps.
There's three main programs you can use to mod at this point, I'll ogo over each a bit.
SmashCommand - Stable Build. Included in the Smash Stuff download, the most reliable and simple of the programs, but struggles with large projects. Recommended at least for starting off.
SmashCommand - Nightly Build. You'll have to download on your own. Like Smash Command Stable Build but you can copy and paste, but saving is harder. To save in the nightly build, you have to exit out of all tabs you have open at the top of SmashCommand, and then do save as.
FITC and FITD - Can be useful if you like using things like Notepad++ to edit. Also amazing for moveswapping because you just have to drop in the right files from the move you're swapping and get rid of the low_voice_clip commands. Also some of the syntax for what each command does is different so it can be hard to tell what does what.
This is the part where you'll have to find your own tutorials. I've given you a start, but you'll have to find tutorials for your specific program. I may make tutorials eventually, but not right now. Hope this was helpful!