You can add dimension lines to your dimension drawing by clicking on the first symbol (the lines with arrows) in the ‘Dimensions’ step under Tools. Once you click this, snap points are shown in your design (provided you have enabled ‘show all snap points’ in your settings. Otherwise, only the snap points you hover over with your mouse are shown).
You can draw a dimension line between any two snap points by clicking on a point and dragging your mouse pointer to a second point. When you release the mouse, the dimension line appears on the screen. You can place both horizontal and vertical dimension lines by clicking with your mouse where you want them to appear.
When you have finished adding dimension lines, click the ‘dimension line’ icon again. You can now modify the placed dimension lines by clicking on them. For example, you can drag them to another place, adjust the length, or add auxiliary lines to indicate to which snap points the dimension line refers. Note: if you want to adjust the length of the dimension line manually, you have to tick the checkbox for ‘length’ first.