PaintTool SAI includes tools for making boxes filled with colors or for drawing the outline of a box or other shape on your image. Unlike some other graphics-editing programs, PaintTool SAI doesn't include a list of predefined shapes you can draw on your image. Use the Selection tool in combination with the Bucket tool to create a filled box or use the Line tool on the a linework layer to draw the outline of a box on your image. The Bucket tool is only available on a normal layer, while the Line tool is only available on a linework layer.
Click the "Selection" tool under the color wheel. The Selection tool appears as a square icon with dashed lines.
Place the mouse cursor over a point on the image for one of the box's corners.
Left-click on the image, hold down the mouse button and move the mouse cursor to draw the box.
Release the mouse button to select the box's outline.
Select a color to fill the box with by clicking it on the color wheel.
Click the "Bucket" tool in the toolbox under the color wheel. Select a normal layer from the layer box at the left side of the window if you don't see the Bucket tool icon.
Move the mouse cursor to the inside of your box selection and click once to fill the box with your selected color.
Click the "Layer" menu at the top of the PaintTool SAI window and select "New Linework Layer." If you already have a linework layer, you can select it from the layer box at the left side of the window.
Select a color for the outline from the color wheel near the top of the window.
Click the "Line" tool in the toolbox below the color wheel.
Position your mouse cursor at a location for one of the box's corners in the image and click once to start drawing a line from this location.
Move the mouse cursor to a location for an adjacent corner. A preview of the line appears on the image; click once to place the line.
Continue drawing the box by clicking at all four corners. Double-click on the last corner to stop drawing the shape.