Click Filter/Noise/Reduce Noise to display the noise filtering dialog. A part of the photo is visible in the preview area to the left. You should view the picture using at least a 100% zoom so that you can keep the filtering procedure in control.
The Settings region on the right contains the three most important sliders in the dialog.
I always adjust Reduce Color Noise first, since it doesn't affect the blurriness of the details, just reduces the abundance of the reddish-greenish spots of color noise. It can have an adverse effect in smudging the rich colors of a picture, just like a washing machine would do. We had to use a 100% setting for the proper result, but you'll have to check the effects continuously on the preview when determining the optimal setting for your photo.
Strength controls the intensity of luminance noise removal. Use it really carefully as it strongly blurs the details in the picture! Available values are 0 to 10, but be sure to check the results of every step on the preview to see how much of the details it would ruin.
You have an aid in preservation work though—the middle slider called Preserve Details. Drag it to the right to make more areas along the edges exempt from noise filtering. For picture areas containing many details, e.g. foliage, noise is barely annoying, as opposed to the blur caused by filtering. Set this slider to a higher value to make the filtering affect only homogeneous areas such as walls or a clear sky, where there are only a few details, but noise is easily spottable. After setting Strength, carefully increase Preserve Details to find the fragile balance between noise filtering and the preservation of picture details.