Robin G. Collingwood is widely known as an idealist writer and yet unlike many writers in aesthetics he does talk about the activity of the artist in her studio. In famous passages concerning "Art Proper" he does this when discussing what it means to express an emotion. The artist feels a certain "perturbation or excitement" which she does not fully understand. She then expresses herself and then, in the process, comes to understand her emotion in a very particular way. At that point the sense of oppression goes away or, at least, is "lightened."
In the section "The Artist and the Community" (Principles of Art, 306-308) we also learn that aesthetic experience is imaginative experience and that this is sensuous experience raised to the level of imagination by consciousness. This happens in a specific way in the studio. He notes that sensuous experience is not present "by itself" since its elements come to be under the artist's consciousness (insofar as he is a good artist) thus converting it to imaginative experience "at birth."
So what are the elements? They include the "psycho-physical activity of painting; his visual sensation of the colors and shapes of his subject, his felt gestures as he manipulates the brush, the seen shapes of paint patches that these gestures leave on his canvass." These are all the sensuous experiences he has before the easel. This sensuous experience must be present for consciousness to generate the aesthetic experience that is externalized in the work.
This transmuting process is not limited to artists. Others who are not artists can also have "impressions transmuted into ideas by the activity of ...imagination." But there is far less here than in painting: the painter gets more out of the experience since he has "put more into it" i.e. in both looking at and painting the object.
All of this is a pretty good description of what goes on in the studio. It is part of the everyday life experience of the artist. The transformation into imagination is a matter of giving the sensuous qualities and objects a new heightened significance, what I called in my book "aura." Although Collingwood is commonly accused of "idealism" what seems to be described here is something quite pragmatic. The activity of the artist in the studio is what gives rises to expression, what is expression. Although Collingwood insists that the work of art is "in the head" it turns out that "in the head" includes this activity in the studio, except that the activity is not just the physical activity of putting paint to canvas but the imaginative activity by which these physical phenomena become "ideas." "There is no question of 'externalizing' an inward experience which is complete in itself and by itself. There are two experiences, an inward and imaginative one called seeing and an outward or bodily one called painting, which in the painter's life are inseparable, and form one single and indivisible experience.." (304-305.)
Useful in this discussion is John Grant "On Reading Collingwood's Principles of Art" The Journal of Aesthetics and Art Criticism,
Vol. 46, No. 2 (Winter, 1987), pp. 239-248.