hi
Very interested in this as well.
hi. it's really complitcated
First you have to download Baldur's Gate 3 modder's multitool
Go to Utilities - Game file operations - unpack game files
Choose Gustav and Shared
Once extracted....you have find the spell you are looking for and this is the hard part
you have to search the name of the spell witch Windows search (or with the tool but i never tried) in the folders you have just extracted. You have to search also inside the files, not just the names of the files.
Consider only .txt files
Then you have understand which is the main paragraph of that spell. usually the name of the spell is in the first line
Example, you are searching for Cure Wounds, you will find a paragraph with first line "target_curewounds" (no space between words..and this is how you have to search for spells)
The problem is that you can't modify much, there. i changed the cost (from action to bonus action) but i haven't yet understood how can i edit the damage (in this case is a cure but..ok)