From Test-Scratch-Wiki
This tutorial will help you draw shapes using the pen using Custom Blocks.
Square
A square is very easy to draw. Use this custom block if you want it in the sprite's direction:\:
define square (size) pen down repeat (4) move (size) steps turn cw (90) degrees end
If you always want it at 90°, then use this:
define square (size) pen down change x by (size) change y by (join [-] (size)) change x by (join [-] (size)) change y by (size)
Your sprite will now draw a square when you run the block.
Rectangle
A rectangle is only slightly harder to draw. Use this block:
define rectangle height (height) width (width) repeat (2) move (width) steps turn cw (90) degrees move (height) steps turn cw (90) degrees end
Your sprite will now draw a rectangle when you run the block.
Circle
A circle is is slightly harder to draw than a rectangle. Use this block:
define circle (size) repeat (round ((360)/(size))) move (size) steps turn cw (size) degrees end
Your sprite will now draw a circle when you run the block.