As bbulkows said, a dremel is one of the best carving tools. The average bit set has at least 1-3 engraving bits. Choose the flattest and smoothest side for your carving.
I use a pen or felt to draw the tunnels and chambers and then take my dremel to it.
Hint: wetting the brick will eliminate lots of dust and doing it outside is even better. Wear safety glasses, if the tiniest bits get into your eyes you will probably be making a trip to the ER to get it removed.
I prefer my nests to lie flat or at an 15-30 degree angle. Watching the ants try to move brood in vertical nests makes me feel sorry for them.
For the reservoir, ytong absorbs pretty fast. You will only need a small tunnel, 1/2cm wide and 2-3cm long is overkill. Make the watering tunnel on top as the water moves down pretty fast. Make it easy to water without moving the nest.
I advise two entrances. One can be plugged, but comes in handy when the ants outgrow their formicarium and need to move to another nest.
Other than that, it is pretty similar to using firebrick.