I don't think you should need to vary the concentration of the GelRed. Mine came with instructions for the exact concentration and how to dilute. Optionally salt can be added, which I did, and it has worked for me.
I have only done this with Post-staining. 1-2.5% agarose with TAE. Used a 10,000x solution from Phenix research. The gelred stock and working solution needs to kept in the dark. I am assuming you have a transilluminator and (optional) filters to visual though. GelRed has similar excitation and emission [wavelength] properties as EtBr, so a standard transilluminator and filter should work without any changes. It will not be visible with the naked eye.
That being said there are a ton of things that can go wrong in casting the gel, eletrophoresis, staining, and visualization. If bands are visible but just blurry I was suspect there is more of a problem with either the gel creation or eletrophoresis though. Make sure agarose is completely clear and cooled down a little before pouring. Otherwise voltage is one of the main things I have noticed that can have significant affects on how sharp or blurry bands appear. You might try different % gels and different voltages. There are formulas that can help you estimate the best value for each of those.