The first time I installed a side plate on an S&W it took me well over an hour to get it right. Use Shawn's instructions as your primary guide with the addition that I've found it best to start at the tab and the front yoke screw as Jepp2 has suggested. This really does work once you "get" the trick of positioning the hammer block correctly so that the slot is engaged on the pin mounted on the rebound slide and slid as far in the "up" direction as the slot will allow. Once all this has been done the slide plate will nest properly with simple thumb pressure.

However, they are fitted tightly enough on some revolvers that you will have to draw them home with the screws. Once you have the sideplate about 80-90% all the way home you are perfectly safe to draw in the rest of the way home using the screw, just make sure you work gradually all the way around the screw pattern using a back and forth technique.

PS; after 3 or 4 times doing this all the stars lined up and the bell went off and now I can put a side plate in place in about 15 seconds without any fumbling at all. Point is, it's only difficult the first few times you do it and once you "get it" it becomes a piece of cake.