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