In the age of digital photography, we are dealing with three different kinds of sharpening. If you develop a RAW image in Camera-Raw, you can sharpen it there, the same if you are scanning image material. This so-called Capture Sharpening compensates for shortcomings of the capture system. Then there is Creative Sharpening, if you just want to sharpen lips, eyes or individual strands of hair in a portrait in order to draw more attention to these areas. And finally there is Output Sharpening, which only takes place at the very end of the image editing process, because it has to be adapted to the output process. Arrange the sharpening with your printer and do a test print if possible.
Sharpening is a change of contrast. After sharpening, light areas are represented lighter and dark areas darker. If you look at the black-and-white gradient, you can see clearly that it becomes sharper and has more contrast after we apply a sharpen filter. But at what price? Details will be lost, soft transitions become hard and quite often the colors are not right any more either. This more or less happens with any kind of sharpening. If you sharpen an image or part of an image, you need to have at least 100% zoom to make sure that one image pixel is represented as one pixel on the monitor. Multiples of 100% are also okay, such as 200% or 400% etc. You need to consider carefully whether sharpening will actually improve your picture.