С пятницы, я провел много часов у сестры в подвале, где я занимался каким-то "художественным произведением" -- т.е., пишу большую картину на простыне, для сестириной церкви.

I mean to say, "Since Friday, I've spent many hours in my sister's basement, where I was busy with some sort of 'artistic endeavor' -- i.e., I was painting a large picture on a bedsheet, for my sister's church."

Specifically, it's a "landscape" (ландшафт?) representing the "sacraments" (таинства?) of Roman Catholicism. For example, a flowing river for крещение (baptism), a table of bread and grapes for причастие (Communion/Eucharist) -- I don't know the names of the other sacraments in Russian, and honestly, I don't really need to know, since I'm somewhere between an atheist and a "Southparkian Deist" (I accept the possibility of a Supreme Being, but I do not rule out that his/her/its True Form is a косоглазый бегемот-кот who gives gay relationship advice to Satan.)

But how would I express "I'm volunteering my time doing artwork"?

Specifically, could I use a construction like "в качестве добровольца"? And if you're only a semi-talented artist producing a large painting on an old bedsheet, can that be considered заниматься художником, or what?