well, lets see if some pictures help. there's really only 2 main steps, finding the offset, and changing the reference.
to find the offset, open the FitCharacter.pac (the PSA file) of the character you're replacing with Tabuu (the program). i'll use Ganondorf as an example, since i just edited one for someone. open the FitGanon.pac with Tabuu, and go here.
the Value is what we're looking for, because that's the HaveN index that the items link to. now, convert that value to hex using the Windows Calculator (or some sort of Decimal to Hex converter). converting 65 to hex gives me 41. do the same for the 0x001 Value, which is 56 for Ganondorf, 38 converted to hex.
now that you have them converted to hex, open the FitCharacter.pac in a hex editor, then use the search function to find the proper reference. when i open the FitGanon.pac in the hex editor, i search 00000041 00000041, because it will take me right to where i want to be, which is here.
i highlighted the section that has the references you need to change. there's 9 spots that need changing, 3 HaveN references, 5 HandN references, and 1 ThrowN reference. the other picture i posted also shows which ones to change from here. the 00000041 bits are the HaveN references, the 00000038 bits are the HandN references, and the 00000045 is the ThrowN reference. change them to match the new model, and don't forget to convert the bone index numbers to hex before entering them.
after you change them to what you want, it should look something like this.
the parts that are red are the new model's references. for this, it's actually a DK model. the 00000036 is DK's RHaveN reference (bone index 54), the 0000002D is DK's RHandN reference (bone index 45), and the 0000003E is DK's ThrowN reference (bone index 62). once changed, save it, and you're good to go.