You're gong to need a .json file as well as a image hosting site in order to make and load your custom character
The image hosting site will hold the sprites for the character
and the .json file load the image into the game
Download the custom character template and use [or make a copy of] the .json file located in the .zip file upon decompressing it, then upload your sprites [images] to an image hosting site [igmur a good option], once all of the sprites have been uploaded copy the image address to the specified area that the image [Player start sprite -> Player Start spot, Front sprite -> Front spot, ect.].
Once all those steps are done, boot up Party Project, click the "Custom Characters" tab, click Choose files and the .json you want to load [or drag and drop it into the game, while in the custom characters tab]