Tutorial Mostly fixed. Any problems let me know via commenting. Also more reliably and quickly you can check out Larian Studios discord and visit the #toolkit-mod-devs-chat channel; here https://discord.gg/qmUJPKuZ to ask for help. Other than that, get the prerequisites and follow the tutorial and you'll be as good as I am right now.

Note: This is a tutorial about using the BG3 Toolkit to override a spell. You are not changing game files, just creating a .pak otherwise known as a "Mod". As stated above the Larian Discord is a good source if you are confused.

This doesn't change the game files, it's just a way to make mods for the in-game mod manager. Basic changes that can be removed by selecting it off the mod manager.

Prerequisites:
Download BG3 Toolkit
(Steam>Library>BG3>DLC>Toolkit Data>Scroll down and click download)
Opening BG3 Toolkit 
(Steam>Library>Open BG3 Toolkit>Enter Game Path it shows… doesn’t work? You probably didn’t download the DLC data)
Steam account
Larian account 
Mod.io account (if you want to publish it to the in-game mod manager)

beginning of tutorial about overriding:
Create Project
Name the Project
Open basic_Level_A
Click Stats Editor                               (To the left 2 times from the white question mark above the little bg3 screen)
Click Shared                                       (only includes spells/spell slots of 1st to 3rd level, sharedDev is 4th - 6th level)
Click SpellData
Locate the Spell                                 (Ctrl f and type spell/a word from it in every option; target, shout, etc)
Right click and “Override” to Project X (Whatever you named it)
go to Project X                                    (Your project you created; mostly likely at the bottom in the Stats 
Spell data                                             
Edit it how you want it (ctrl f)

example (based off my   "PlantGrowthRitualLR" you will be editing Plant Growth to just be a ritual cast spell (No combat, infinite casts) and Last Long rest (Doesn't disappear; can't LR in the level)

Stats editor path for my mod:
PlantGrowthRitualLR>SpellData>Target>
Now edit Plant Growth                                                                             (below I will go over the letters; do what you want and test around)
d spell level                 |  3                                                                          (don't change; this is the level of spellslot used/seen as)
k spell properties       |    GROUND:CreateSurface(6,-1,Overgrowth)    (-1 is Long Rest, 10 in place of -1 is 10 turns (normal). the 6 represents radius
n target radius           |   18                                                                       (How far away you can cast it; around 30-40m I think thats the limit)
o  areaRadius            |   6          (6 meter radius)                                (radius in meters; you can change it to 30; check Better Plant Spike Growth 2C example image
AB display name       |     plant growth                                                (If you want to change it for whatever reason; knowing this exists will help u cr8 new spells
AC description           |     description of spell                                     (^)
BH UseCosts             |    ActionPoint:1;SpellSlotsGroup:1:1:3           makes it cost 1 action; alreadu implemented base game 
^Note: instead of shared, in shared dev>spelldata>target> Ctrl f "plant growth" and see PlantGrowth_4 (upcast version) and see how it has ActionPoint:1;SpellSlotsGroup:1:1:3 still meaning that it has no upcast benefits. Compare with IceStorm (ctrl f) and look where it has upcast changes in 
AI tooltipdamagelist. Now you know how to determine possible upcast changes, other spells have different ones, hope this helped.
BK RitualCosts          |    ActionPoint:1                                                 not in combat? Ritual cast (infinite casting). To remove just delete/ make empty


That's mostly all for that spell, Test if it works by clicking the Switch Game/Editor button at the top left below files, that has a little controller icon.
Useful commands:
Ctrl+Shift+11 = command menu       (has other name but I forget)
type "ccStartRespec"                            Respec your current character    
type "levelup x"                                     I recommend 4 for x, as it gets you to level 4 for testing plant growth

Done? and want to publish your own mod to share with others or easy use with your friends? Make sure the toolkit is open and follow these steps:

Top left of the screen
Projects
Project Settings
Fillout: Mod name, Author (you), and Description.

click upload local
Choose mod folder file path C::\Program 86\Users\YOURUSERNAME\local\localappdata\larian studios\Baldurs gate 3\mods
Tip: DO NOT EDIT THE FILENAME THAT AUTO FILLS; it prevents you from publishing to mod.io (I figured out myself ;/   )

Click Publish
Login/ connect Steam-Larian-Mod.io account              (Mod.io website will pop up eventually)
Wait for file to be scanned                                               (on mod.io)
Click the Edit Pen                                                              (on mod.io)
Add to your platform / all                                                 (on mod.io)
You are done! Congratulations. 


There are more guides on mod.io here for the toolkit, some about creating new spells, feats, classes, etc. I recommend them.