Reshaping and polishing a scratchy nib so that it glides over the paper is an art. The principles may be simple (aligning the tips, rounding and deburring edges and sharp points that would otherwise dig into the paper), but much patient practice is necessary to become truly adept.
Unfortunately, we haven't the time to give individual instruction on nib smoothing, especially given that many of the techniques are not easily explained in writing. Fortunately, helpful information is available in a number of repair manuals, including the various reprints of factory originals available through the PCA Reference Library. Much useful material is also to be found at John Mottishaw's nibs website. Online, there are an ever-growing number of reference sites to be found by searching under "nib smoothing".
Learning to smooth your own nibs is very satisfying, so don't be put off trying. An expert may be able to get a nib smoother, but that doesn't mean that significant improvement isn't achievable for the amateur, even on the first attempt. This is particularly true with newly-made fountain pens, so many of which have nibs which are properly ground yet lack final tuning and tweaking before leaving the factory.