Confocal: Merging Fluorescent Images in Image J

How to perform typical steps for confocal and fluorescent images from three channels (red, green and blue) to merge (overlay) these together into one image. Typical fluorophores incluce DAPI, FITC, and CY5 or the ALEXA dyes. Image J is a free program: a more robust version is also free and is called FIJI.

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Thank you ! It is really helpful !!!