I am having such a difficult time moving any of my main saves over to custom games in order to try mods on already established characters of mine any help would be appreciated in how this is done I’ve looked it up over and over and tried the different methods and I simply have no clue how to do this…
Use Mamba’s GD stash to do it.
I’ll elaborate a bit here as it is a manual process, not automated 
Get GD Stash, as mentioned.
Copy your main chars (entire dir) to the user dir
Open the char in the user dir and set its Mod-flag, then save that change.