Playscii is a very versatile tool to create not only different kinds of ASCII art but also animations. We'll focus only on the part of creating ATASCII here.
You may watch and/or follow the description after it.
After starting the editor press:
File -> New
Select a name.
Leave the Width and Height.
Remember the save folder!
Press space so that the charset/color picker pops up.
Select the correct 'AtariAll' character set. (If you can't see this option you need to install it.)
Select 'bw' (=black/white) as "Palette: ...".
Now you should be able to paint.
Click on 'File' -> 'Export...'
(Do not use 'Export last'. This option does not work always.)
Select the 'ATASCII XEX' exporter. (If you can't see this option you need to install it.)
Chose a file name and save. An executable xex file will be created.
We recomand the Altirra emulator.