Not much really you do the same for a turbo head as an NA head. Except cooling should have more time spent on it and it shouldn't be skimmed to much.
Engines all work in the same way (except rotaries) pistons go up and down make power in the same way. With a turbo you just happen to have a bit of force shoving the mixture in. A bit of a sort out will mean more air can get in with that shove.
Also same as any head you want the ports to flow equally.
I've been told that it is more important to port the exhaust side to make the best reduction in back pressure and jsut equalise flow in the inlet ports, and most impoortant was to match comustion chambers.
If it flows better then less boost is needed for equivalent power, right??
the main difference is that the exhaust side on the turbo head can be ported right out (within reason) without fear of making it susceptable to reversion - which is to be very much avoided on a NA head.
A couple of mill (I'm talking rubbish), a few good mill it should read
Cam timing for turbos is milder, so reversion appears to be less of a problem. However, the FLOW figures involved are much higher.
Off boost is also affected, and boost buildup time is less with a nice step there. Remember that off-boost the turbine acts as an ugly, useless flow-restrictor - trying to keep the burnt gases in the chambers
As people who do these mods tend to fit larger turbos anyway, minimising these losses help keep some of the 'stock' lowdown pull.