- Select your drawing tool,
- Put your mouse cursor where the line should start (if you have already used it, it will start at the last point of your last action),
- Hold the key SHIFT, and move your mouse to the end of the line,
- Click to draw the line according to the displayed guide.
It’s that easy ! And if you want to draw a straight line with a fixed angle (like horizontal or vertical), hold the key CTRL on top of shift, only few different degrees of inclination will be allowed.
For an arrow, simply repeat the operation thrice.