The Hero Printer, right? There is a file named NEWASSETS.txt inside the ZIP, it describes the whole process with an example. It is nothing too hard, just requires replacing/adding a couple of files on the right folders and reloading the tool :)
EDIT: Let me know if you run into any difficulties after giving it a read and trying it out.
Okay. I might br doing something wrong then :( i tried to make a copy of hat for example and named it 07 if the other one was 06. I even tried the new_hat or something along those lines. I'm not sure what I'm doing wrong. I'm sorry for thr constant messages. Also am I able to add custom skin as long as it's 48×48 max? Thank you again.
Oh! There is one thing you can try.
I am assuming you are doing only the 3f version, right? Place an empty file, with the same name as your new hat on 3f, under assets/charsets/6f/hat. It always lookup at 6f first and builds the list from there, there is just no need to have anything on that file, it just has to be there, even if it is a blank PNG.
And yup for the skins. Remember that, if you are also doing facesets there are some extra steps to get everything right for the facesets as well!
EDIT: Keep in mind that the 6f being verified first is also true for skins, so if you are doing only 3f, remember to create an empty file on 6f as well, with the same name as your skin on 3f.
