Well, that's quite easy actually

.
See this post of mine, I recently have done a good job using this method: (see description below link)
First build thread 'Roberts JB4'
- Clamp two bars on each side to the neck.
- Use a ruler to check if it aligns with the body.
- If it aligns okay, clamp it to the body.
- Stick a small piece of wood on the body with double sided tape and secure it with another bar and a clamp.
- Remove the neck.
- Start routing.
And it fits like a glove!
(However, I did it over in a later post to make the pocket deeper, but that doesn't matter concerning the method I used.)
Edit: I forgot to mention in the list of steps that you need to clamp the bars on the end of the neck, beneath the neck, too. As you see in the pics. This, together with the clamps on the body, make sure the bars won't move at all after removing the neck. Of course, when everything is fastened you should remove the clamps you've started with (in first pic) to be able to remove the neck.