It's kind of obvious where the forks inside the transmission are supposed to fit - that is, assuming that you do have a clue as to what you're about - the fork "straddles" the selector hub and fits into a groove on it, so that when it moves, the hub moves with it.
It's also one of the things that most folks check with the transmission on the bench, before reinstallation - slide the shift rods backward/forward with a large screwdriver to engage the various gears one by one - you'll feel the hub snap into position - 1/N/2, 3/N/4, 5/N/R.