Not necessarily all the way back on the board. You see, depending on the dimensions of the longboard, with thickness, length, and everything else factoring into the radius of the turn that the board can make with a lean on the rail, that's potentially a VERY slow and VERY wide turn. In other words, my 10' Dewey Weber is a "bulky" board. Excessive in all dimensions, making it a real bull, but very fun to walk around on. If I was to take a spilling wave and WITHOUT shifting my weight towards the fin, it would take me a very long time and a very large radius to achieve a 90 degree shift in direction (going perpendicular to shore, to parallel in either direction; the right or left). It's just the hydrodynamics of these types of boards. Now, with my 9'2 Stewart (tri-fin, fast longboard), it wouldn't take quite as large as a radius as the Weber, but it's still significant if you're looking to shoot down the line sooner then later.
So now you can't just lean into the board and have it dig in, catching a rail and shifting direction. What you have to do is move your weight closer to the fin (in the case of my Weber, it's VERY close; not so much with the Stewart). If you can visualize a longboard turning, it doesn't "list" (go up and down, port to starboard, left to right) as much as a shortboard will. The way a turn is achieved is through using the fin as a pivot point on the horizontal plane. It would be ideal for a turn to have 100% of you weight right on (above) the fin, but this is not ideal for speed (would end up stalling WAY too much, depending on the board and conditions). So get as close to that fin as possible without losing stability and speed, and just push out your front foot to the direction you want to go, and use the back foot as the "fin pivot" foot, keeping that plant, and also applying some pressure in the opposite direction.
You'll get the feeling of it, but that's the general approach you should have and the thoughts going through your head. Remember, these boards have very different characteristics; their size and dimensions makes them want to stay in the direction they are already going, making you have to coerce it a little with foot trim and a little pressure.
I didn't explain the above very well, and it turned out to be a novel. Let me know if it isn't clear and I'll try another way at explaining.