In Illustrator, shapes can be drawn either edge-to-edge or from the centre outwards. Since we need the circle to be centred on the intersection point of our two guides, we must use the latter method.
Click on the oval tool on the toolbar.
Position the cursor (which will change to a crosshair) over the intersection point of the two guides. When the cursor is accurately positioned, it should change colour.
Hold down the Shift and Alt keys and drag diagonally outward from the centre to create a circle centred on the point you initially clicked on. By holding down the Shift key, you ensure that you are creating a circle. By holding down the Alt key, you force Illustrator to draw the circle from the centre outward.
Set the fill and stroke attributes of the circle as desired. In this example, we have chosen a radial gradient fill.
