You have to go to the page https://silverfeelin.github.io/Starbound-Hatter/ there you can take a picture with Paint 3D or another tool which, as stated there, must be approximately 43 by 43 pixels. You shouldn't forget that the background has to be cut out, which is very easy with Paint 3D afterwards you insert the picture there and generate a spawn command which you can activate in the game in the console