You can create a rectangle in three different ways with the Rectangle tool in the Sketch menu:
- Based on its center point
- Based on its diagonal line
- Based on three points
When you select Rectangle, choose the type of rectangle you want to create:
- Center
When you select somewhere on the grid, that point becomes the center of the rectangle (and an anchor point, which is indicated by the X-mark) and you can drag the shape to create the sides of the rectangle. So, you’re defining the rectangle by its center point.
- Diagonal
When you select somewhere on the grid, you need to drag an imaginary diagonal line. This line becomes the diagonal length of the rectangle, and the initial point becomes the anchor point.
- Three points
You need to create two sides in a continuous motion. The remaining sides of the rectangle are automatically created based on the two sides, or three points you specified. The first point you created becomes the anchor point.
When you’re done creating the rectangles, you can finalize the length of the sides. Note that changing the length of a side means that the rectangle, as a whole, will adjust—according to its type and anchor point:
- Center
- Diagonal
- Three points
If you don’t want the entire rectangle to adjust, then disconnect the sides with the Disconnect constraints, just as you would for any other polygon. So, select the sides and then Disconnect. Notice that the edge points are no longer filled. 
Watch this tutorial: