Traditionally I start painting titans from the inside out, meaning I do the interior first and then work on the outsides.
In the picture above you can see the insides of the frontal part of the main body and the cockpit in the head with the tech priest as well for size comparison.
The interior is pretty much done now, all that's left to do is some work on the silvery metal stuff. Some weathering, oil spills, rust, etc. But that's a job concerning large parts of the model and I'll do that all at once. Or at least in larger chunks.