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Here are a few tips from a guy that has used a lot of Spanish heel neck joints.
Some people cut those slots for the sides oversize (as much as 1/4") on purpose, fit the side into it, then custom shim the inside of the slot to press the side out tightly against the heel. I haven't tried that yet, but it would really simplify getting this joint nice and tight.
Otherwise you have to cut that slot accurately to begin with. I cut mine on a band saw with a nice sharp new blade (so it doesn't wander). It lets me cut that slot as the gently curved cut that it needs to be. I carefully mark the slots out on all four sides of the blank with knife cuts, not pencil lines, for the width of the slot. Prerequisite no. 1 for cutting slots this way is that the neck blank has to be really square. The bottom of the neck has to be parallel to the top surface of the neck. Unless you have a very large band saw, you have to cut the bass side slot with the neck facing up and the neck jutting outwards from the saw (away from the column), then cut flip the neck blank over and cut the treble side slot with the neck facing down. (The column of the saw is in your way otherwise). Any error and the slots don't line up at the heel, where it is most obvious. Prerequisite no. 2 is that the saw has to be set up to be cutting right at 90 degrees for the same reason. Take a few minutes to check the saw set up. Also check the front of the blade for verticality . (You might have to crank the top wheel tracking alignment screw to set this). This cuts a slot at about .060". Cut between the knife lines. Then I carefully draw a steel rule with varying grits and thicknesses of sandpaper through the slot and widen (and smooth) the slot to about .080". Leave the neck blank overly wide for this as the outside edge of the slots will get over rounded. (The the rounded part gets cut off in the waste later). Since I make my sides usually at .085", it is a matter of slowly scraping/sanding the last inch or so of the sides (down to .080" )to fit in there until they slip in easily yet fit tightly. It also helps the sides slip into the slots more easily if they are sanded nice and smooth. Round off the end of the side too. (As a bonus the sides are already smooth in the area up against the neck when you are ready to sand the body of the guitar later for finish prep and you don't have to fuss with sanding too much at that joint area.)
Then, with a good tight fit, you will have trouble if you try to glue the sides in with any kind of water based glue because the wood surfaces will expand when the water hits them. The result is that the sides will lock up on you before you can get them positioned. I solved this for myself when I switched to a slow set epoxy for this joint. Slow set glue gives you plenty of time for alignment and is like a lubricant during assembly. I only apply the glue to the inside edge of the side/slot as the epoxy will leave a visible dark glue line between the neck and side. With a good tight fit that's all it takes.
After writing all this out, the first option above, with the oversize slots and filler/shim, is sounding much simpler. I'll have to try it that way soon.

Sorry, I meant to post this as a response to filling neck joint question in the main discussion....
I'll get the hang of interthingy this yet...

That shim was exactly what I was talking about. Using wedges on the inside snug the sides up perfectly....no fuss with fitting if done correctly. Hey Rob....I think we live a few miles apart!

~Brian

Brian, where exactly are you? Do you attend Seattle Luthiers meetings?

Hey Rob

I am on the east side in Renton. I have made a few meetings. I was at the one when Fox was there and you played the Ergo guitar for all of us.

I show an easy way of cutting the slots for a Spanish heel on a table saw in my classical guitar building DVD. I also have footage of this and it is on my long list of Luthier Tips du Jour videos to produce. I need more time in the day!!

Like Rob i use the table saw also!

I have a carbide blade that cuts a .084 slot.
It's perfect and using the table saw is very easy!!!

Mike

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Mike Collins