Avoid background noise. Use a good microphone. Higher sample rate creates larger file size. Leave at least a second of silence in the recording to aid in future noise reduction. Make sure that the wave is centered before applying any other transformation.
Quick filter works like an equilizer on a stereo.
FFT filter allows notching out specific frequencies e.g. for noise elimination. Turn the volume up high and listen to the silences in your samples to get a feel for what the NR algorithm is removing. In fact, the noise is not really gone. It will be replaced by a quieter sound. NR can also introduce distortion and can remove some signal along with the noise.