First Learning Smiths:
Before even trying Smith Grinds you will need to know your basic grinds first; 50-50, 5-0, B/S + F/S Boardslides , mabye F/S Lipslides.
A Smith is in between a 5-0 and a F/S Lipslide. You have the back truck on the rail and the front truck should hang down a few inches. You might want your board to touch the rail. Just let it hit in about the middle like for a Boardslide. Make sure your front wheels don't hit the ground. You dont want that.

Learning Smiths:
Ok, now for the real tip. First learn these on a flat rail or round rail 8-10 inches high. Boxes screw it all up. You'll need some speed for this so go kinda fast. No more Granny speed. Start your ollie and turn like for a F/S Lipslide, but only turn a little less than 45 degrees. Land on the back truck and start to balance like a 5-0, point your front trucks down. Do not lean forward. Keep your weight the same as you would for a 5-0. Make sure you point the front truck down a little lower than the surface of the rail. It's kinda hard to keep your balance without doing a 5-0 or a front truck touchdown. Come off of the rail like for a 5-0. Just practice these a lot. Do a few 5-0s and Lips on your rail for warm-ups. I was skating up town with some friends when I learned this. We brought a rail and I just charged it. 3rd try, I landed my first Smith. Practice, practice, practice. Remember to have no fear when doing this. You wont get hurt so stop acting like a little baby.