Idea! This is what I did and the sides of my neck pocket are so straight and tight I have to fight to get the neck back out.
You first create a template or take the neck you are going to use, and lie it down on the body and trace an outline. Then take a 2X4 or any other straight piece of wood. Line the edge of your router bit up with the inner edge of the line you drew from the neck. Leave the router in the exact spot where the bit lines up with your outline, and set up the 2x4 along the base of the router going parallel with your line. Clamp the 2x4 and guitar down to a sturdy table and take your time. When you rout along that 2x4, you can keep a straight line and you will have a nice neck pocket.
Since you are not yet well versed in the usage of a router, I would get a cheap plank of pine or something and use it as a test for this method. Give it a shot, I found it quite useful.