The traditional or spanish way of joining the neck and body involves cutting two slots in the 'foot' of the neck where the sides are subsequently inserted. This way the body is actually constructed around the neck, one of the advantages being that the whole assembly is very solid; another is that you can control very well the alignment and the angle neck/body.

There are a few methods used to prepare the slots for the sides: one that i've used for a while is to cut bigger, wedge-shaped slots, then insert the sides and make suitable wedges to fill the slots and secure the sides in place. Although once you get used to the process it does not take forever to do it this way, still is a much longer process than the following one and although if done right the sides are held very firmly, that's also true of this other method.
How i do it now is to simply cut two slots of the same width of the sides (2mm) on the table saw.

You need a proper blade, with the right width of cut, and preferably with a straight tooth every 3, so the base of the slot is flat and not 'v' shaped.

With the aid of a fixture i can control the angles of cut perfectly and achieve a very neat and clean result.

In order for the cuts to be correct, the fixture have to hold the neck angled lengthwise and also across the width; it takes some planning to figure it out and to make it precisely, but once you have the fixture made and everything is setup properly, it is just a matter of seconds.

When i thickness the sides then, i leave them .1 or .2 mm thicker (2.1/2.2 mm) and then proceed to scrape slowly the end to insert, until it slides in tightly.
I find this to be a very effective and precise way of achieving this crucial fit.