This page needs a shorter title, ngl.

Standard Disclaimers

1. This won't be a simple start to finish step by step guide. I'll be directing you at other guides for details like modding your switch to set up mods, using setting up emulators, and dumping your game.
2. Do stuff at your own risk. If you run any of this on your switch and get banned, that's on you. If you do this and somehow break your save, that's on you. If you do this and your PC catches on fire for whatever reason, that's still on you.
3. Do not ask for where to find any kind of roms or whatever.

Prerequisites

You need the following:

1. Basic Reading Comprehension.
2. A modded switch or a properly set up emulator with a copy of MHGU. I used Ryujinx for the emulator, and use/recommend Atmosphere for switch modding.
3. Karameru to extract and edit .arc files.
4. (optional) a copy of mhxx if you want the mhxx-only files. Do NOT ask me for them.
5. (modded switch only) NXdumptool
6. A hex editor. I use and recommend HxD. It's free, it works, and I'll be using screenshots from it.

The Process

Weapon Transmog:

1. After you set up your emulator or modded switch, dump the romfs part of MHGU.
2. (optional) if you want MHXX-only content, also dump romfs of MHXX.
3. Using Karameru, extract the table folder (or just the files you need from it) from resident.arc, which should be located in 0100770008dd8000\romfs\nativeNX\loc\arc of your MHGU romfs dump.
4. Open up the weapon??BaseData file corresponding to the weapon you want to edit in HxD. You can find the weapon ID to use instead of ?? here. For example, for greatsword, you'd want the weapon00BaseData.
5. Set the bytes per row in HxD to be equal to the number of bytes per entry for the weapon type you want. 32 for bowguns, 33 for GS/SnS/Hammer/Lance/LS/Bow/HH, and 34 for CB/IG/GL/DB/SA.
6. Go to MHGU or MHXX kiranico and find the weapons you want to transmog from and to, then use the inspect element feature in your browser to get their IDs and record them.
7. Search for the weapon model ID in hxd, and make sure that the result you get is in the 13th and 15th (labeled 0C and 0E by default) columns, the ones responsible for the weapon's model and sound.
8. Change the ID to the ID of the weapon you want your weapon to look like.
9. Save the weapon??BaseData file in HxD.
10. Use Karameru to replace the original weapon??BaseData file in resident.arc with the edited one, then save your newly edited resident.arc
11. (optional), if you're using a weapon that's not in MHGU (USJ weapons and frying pan HH), copy the weapon model .arc file from your MHXX romfs dump to your mod folder, and make sure it's in the same structure nativeNX\arc\weapon
12. Apply the mod and have fun.

Porting MHXX armor

1. Do steps 1-3 from the Weapon Transmog part
2. After finding the armor IDs from Kiranico or via doing a comparison between your MHXX and MHGU dumps, copy the MHXX-only armor folders from nativeNX\arc\player\com\m for male armor and nativeNX\arc\player\com\f female armor to your mod folder, keeping the same folder structure.
3. Open up armorSeriesData.7896B60A in HxD, and set the bytes per row to 127 bytes.
4. Look for the value of the model of the armor you want to replace, I'd recommend something simple like LR Bhrena, which is very early. It should be in the 19th and 20th columns (labeled 13 and 14, because the label values are in hex).
5. Change the value to the ID of the model of the MHXX only armor, which should be the same as the folder name\number you copied in step 2.
6. Save armorSeriesData.7896B60A in HxD.
7. Use Karameru to replace the original armorSeriesData.7896B60A file in resident.arc with the edited one, then save your newly edited resident.arc
8. Apply the mod and have fun.

Example for weapon transmog steps 5-8:

Using weapon00BaseData (greatsword) for demonstration purpose. The goal would be to replace the golden surfboard (ahtal GS) with the clearly much superior looking Palette Knife.

Step 5: Using weapon00BaseData (greatsword) for demonstration purpose. GS is 33 bytes per entry, so I set the bytes per row to 33.

Step 6: From Kiranico, Ahtal's GS' model ID is 181 and Palette Knife is 190.

Step 7: Using the search option in HxD, I find 181, and it's in the right columns

Step 8: I change the 181 to 190 for both the model and sound efx.

Proof of Concept

USJ armor replacing LR Bherna. USJ weapons and frying pan HH and guitar SA replacing petrified weapons, and second USJ LS replacing obsidian LS:

https://mega.nz/file/I9RxhSoY#T73AGWWi3vnzXv_IuVsxmSACmV7HusHxRc-T3NnOwVg