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 standard Amstrad ASCII here.
After starting the editor press:
File -> New
Select a name.
Leave the Width and Height. (For mode 0 set the width to 20, for mode 2 set it to 80.)
Remember the save folder!
Press space so that the charset/color picker pops up.
Select the correct CPC character set. Mostly you will want to chose CPC Mode 1.
Select 'cpc' as "Palette: ...". (If you see a gray color after the white color, then you should either ignore this color or install an updated color palette.)
Now you should be able to paint.
Attention: For Mode 1 use only 4 colors, for Mode 0 not more than 16 and for Mode 2 only 2. If you use more colours then the excess colours will be ignored.
Click on 'File' -> 'Export...'
(Do not use 'Export last'. This option does not work always.)
Select the CPC exporter. (If you can't see this option you need to install it.)
Chose a file name and save. An executable bin file will be created.
Otherwise put the resulted bin file on a disk (.dsk) and run it in any emulator. To put it on disk you can use a tool like ManageDsk.