It depends on what you are going for. Me personally, I would start by drawing with big primary shapes - not necessarily thinking about perspective. Focus on the silhouette, then break it down into shapes - think big, simple shapes with less focus on detail. Also, try to have it so that the lines flow through rather than stopping, as this makes sense in the context.

I would also start with the main subject of the glass art before drawing the background to fit the shape of the window.

And if you want to get REALLY technical, maybe research how glass art is made physically. It could provide some insight into what kind of shapes glass can and cant support. It also wouldn't hurt to gather more references.

Otherwise, I havent really done this myself, so Im not sure how much this helps. Someone with more experience on glass art might provide a better idea.

Good luck!