If you started with a low-res bitmap you will get low-res results.
You will have to convert to grayscale and clean up the image preferably in photoshop. Using levels and unsharp mask. Remove noise is good too as you can remove the jpeg artifact.
Now just print it to a piece of paper and see what you have. Is the black 100% is there any dirt?
Go back in and get rid of what you don't want. If the black is not 100% select the layer and select "select pixels" from the layer drop down menu (layer cannot be background layer). Go to edit "fill" and select black 100%.
Now print it. Assuming it looks right print the real thing as follows.
Go to print select output. Select screen. Turn off default screen. Change lpi to 50. Screen angle 22.5. Ellipse dot. Select accurate screens.
That's the gloss over of this subject to keep it simple.
You will need a 230 mesh screen to hold the image with no moires.
The rule is lpi x 4 + 1=mesh 50x4=200 +1= 230 mesh.
If you are doing in Corel it's the same the menus are just in different places and you would be using Corel PhotoPaint.
I don't use dot gain settings anymore as I know what I am going to get when I see the image on the monitor and make my adjustments. Common dot gains are 20% when printing on an auto, 30% when printing on a manual.
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