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.
Export
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.
You might want to use Graph2Font in combination with MADS Assembler to create coloured ATASCII artworks.