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There are a lot of different ways to draw simple shapes in Photoshop, for example the Pen tool, or even the built-in Shapes tool. However, the Pen can often be time consuming, and Shapes has a limited choice. The best solution is the Polygonal Lasso tool.
This enables you to draw freehand, while restricting your movements to straight lines. This allows for the crisp, neat lines of the Pen tool, but with the speed of custom shapes. With the Polygonal Lasso tool (L) selected, click to place your first point, and simply move your mouse and click to place your next. When finished, either re-select your original point to complete your shape, or double-click. This creates a selection. Fill this selection using the Paint Bucket or Gradient tool (G). Use separate layers for each element of your poster.