You have to create a new project first. Open SAP, click File / Add Save / Save and confirm "Create a new save...". You can access the files with "Open Folder". There you will find your new save with the font_override file to replace. Don't forget to change the tile size value in the json file. When done, you can go back to SAP and press Load after selecting your project. It should load the new bitmap! If you only see a black square, change the paint color to white. You can find more info under "Help".
Right now it only works with one tile size for all tiles and only one bitmap/tileset file. I know that this is very limiting...
