For increased accuracy use the following settings in the Drawing Settings:
Snap to orthogonals provides the ability for your sketched strokes to snap to the vertical and horizontals - creating perfect 90 degree corners.
The Show dimensions shows a temporary dimension as you sketch so you have feedback on the size of your sketch. As strokes intersect we only show the dimensions between these intersections:
Enable vector editing switches the editing mode from sketch-based to vector-based. Refer here on detailed usage of the vector editor.
The Show guides will display some editing guides during editing. These guides also show when you are creating 90 degree corners, even when the edges are not horizontal or vertical.
If Snap to orthogonals is selected as well then all items in the view will provide horizontal and vertical guidelines allow you to snap and/or align points when editing or when dragging/rotating items.