This will be short and sweet.
Pretending this is a coloured drawing ready for toning. Make sure you image is set to grayscale mode and not RGB or CYMK. For each gray you wish to have a different sized tone, colour it on it's own layer. Naming the dot sizes you want will help you keep track. Also when you start you image make sure the DPI or Resolution is set to either 300, 600 or 1200.
Go to Filters>> Pixelate>> Colourhalftone and something like this will appear
Channel 1,2 and 3 are RBG. Red Blue Green. The numbers you place control how Red, Blue and Green line up with each other. So in theory. If you place channel 1,2 and 3 all with the same number, for this instance 0 and leave channel 4 at the angel of 45 you should then get something that appears as such.
For me working at an angel of 100 degrees for RGB works great but that is what channel 1,2, and 3 do. They align RGB and the 4th is the overall angel and alignment. 45 degrees is the standard toning angel which suits most of our needs. Below will be channel examples of settings: 0, 100, 25, 15 and 30 to show you the other ways to achieve screentones in other angels and alignments using channel 1,2 and 3.
Where it says Max Radius: 8
That simply controls the size of the dots. Small number, small dots, big number bigger dots in theory. I have had some unexpected things but this is the basics of screentoning using photoshop. Amoung filters you can use noise and mezzotint for textures such as material like you see in manga. Don't let the zoom fool you either. Sometimes zooming out shows you funny jagged dots but they're perfect circles. Not sure why zoom distorts it like that. Takes some getting use to.
Halftone is not for manga. It's actually for magazines and newspaper printing. Anyway I hope this helps some of you out there with screentoning in photoshop.