open aperture widest possible
Only if Ce_Jeu wants a shallow DOF.
set iso to 800
This will likely introduce unwanted noise to the image.
half press the shutter
In low light, autofocus may not be able to pick out a point of high contrast.
The camera will calculate a shutter speed that it thinks will give a good exposure
Unless it reaches the >30s range, in which case Ce_Jeu will need to switch to manual and set the shutter himself.
adding light with a flash
The best decision - although an external flash will produce far better results than the built-in one. Get a diffuser for the flashgun and a cable connection for it - you'll need to bounce the light off a wall onto your subject in order to avoid harsh shadows. Ceilings and walls are good for this. Failing that (you may be in open space) get yourself a reflector and fire it onto that.