Flatten chosen thumbnail, increase it in size to maximum dimension you need.
Do you need to try a different approach? Things don’t always come together right away! If it’s not quite right, experimenting at this stage is cheaper than changing later.
Do line work, add detail so that you know what to paint
Think of it as thumbnail, version 2, not final. You can always re-work it.
Do you need to consider your eye-level? Be sure your eye-level is clear, even if there’s no horizon.
Do you need a perspective grid?
Are your silhouettes clear and distinctive?