The quick and easy way to find the edges for placing will cost you about 50 cents. Buy a pad of the very small "sticky notes" and store it with your filters. Before putting the filter into the holder, place one of these sticky notes so that the edge marks the boundary on the filter. Now you can see it easily, but you have to remember to remove the note before taking the shot. The adhesive on the paper does not leave a mark on the filter.
As for choosing between hard and soft edged filters, I let the scene decide for me. If the horizon is forested or irregular I use the soft ones. For strong horizons I use the hard edge. I use my light meter to determine which density of filter to choose.