It's not hard, just tedious.
There should be two guide that go into groove in the helical. Those need to be removed, through the mount and the helix will screw right back in.
The tedious part is twofold. Everything has to line up to get the guides back in place and setting the focus. There's one lineup that will do this and the method
is, assemble, check focus at film plane, check guide, curse, try again using a different starting point, assemble, curse etc.