The group portrait is coming along ok so far. This is the stage where the figures look like the Nazi's in Raiders of the Lost Ark when the spirits from the Ark blast through them! A bit freaky!
On a more serious note, after drawing a rough layout in graphite to determine the placement of the faces, I go straight in with the acrylic (not worrying too much about colour at this point) and start to bring out the forms, the curves and shapes. In my mind I'm constantly measuring between things... the outside corners of the eyes are the same distance apart as the bridge of the nose to the end of the nose (in one case). I'm also measuring and comparing the negative, empty spaces.