With this step out of the way we can now get on with the painting process. So the first thing to do is set this layet to multiply and then create a new layer and begin by very roughly blocking in areas of light and dark and try to establish early on which parts will be in shadow and which will recieve highlights (see the image below). The vents will naturally appear very bright compared to the rest of the scene as these are the light sources. The fact that there are no other windows or entry points will serve to emphasize this and so give them an even brighter quality not unlike clear glass windows in a church or cathedral where we can observe an almost blinding light compared to the dimly lit interiors.
Remember not to be too fussy at this stage as we are only blocking in main areas and are not intending for a finished look. It is quite a good idea to use a largish brush for this stage and maybe a watercolour one so as to encourage random accidents and discourage any attention to detail. For this stage I chose a standard "watercolour small round tip " brush and altered the settings to those similar to the image on the left and on the right. As this will be a stone corridor I selected a brownish colour to block in the dark areas and did not worry too much about staying within the guidelines.
Step 4 :
With this initial stage underway I then went on to use the same brush set to a smaller size and add some additional darker areas that this time have a more scratchy look compared to the previous brush. I find that at this stage it is a good idea to freely apply marks and sweeps of the brush and try to let the medium suggest some of the results in a way similar to drawing on paper (see the image below). I really enjoy charcoal drawing for this very reason - one can create large tonal areas to forge out the lighting as well as drawing in fine lines and shaping form in a variety of ways but also allowing for happy accidents as it is not as precise as a pencil. This versatility can be mimicked in a digital format through utilising the large array of brushes availiable in Photoshop as well as customizing existing ones. Some of the random thinner marks may be used to suggest cracks in the stone or just general weathering.