If you want to get good at this, you first have to realize that there are many different types of sepia toning.
For example, you can first convert an image to B&W and then give the result a sepia tint; you can leave it in color and then add sepia to the shadows but leave the whites as white (ie, not sepia); you can do the opposite, ie, color the highlights and nearly burnt out areas with sepia, but leave the darker areas either in B&W or their original color; you can tint only the mid tones and leave the brightest and darkest areas as they were; you can do mixtures of the above; you can add any of the above to the edges of the photo, but not the center, etc. etc.
They all convey a different sense of the scene / subject, different emotional impact (eg, old-timey vs romantic). The only way you will be able to intelligently decide between them is if have experimented with each and see which you like for different applications. There are a zillion tutorials, plugins, PS actions, etc. available to help you do these. None of the techniques require advanced PS skills.