Make yourself comfortable in a sitting position, with your spine really straight. Relax yourself, loosen the tension in your neck and slacken your jaw. Then close your eyes.
Breathe in deeply "through" the Root Chakra, i.e. imagining that the air is entering via this chakra and then passing through the 2nd, 3rd and 4th chakras and revitalising them, before filling your lungs. Then exhale "through" the Crown Chakra, imagining that the air you are expelling from your lungs is stimulating the 5th and 6th chakras en route.
Transform the air swirling out of the Crown Chakra: imagine, visualise this air changing into bubbling water which gradually becomes still.
Visualise, floating on this water, a beautifully pure, white or pink lotus flower. Place yourself in the middle of this splendid lotus flower.
If you can, draw a picture of this image, which you will then use as a meditation aid for three days, to "anchor" your feelings relating to this visualisation. To begin with, you may only have partial success with visualising (i.e. creating your own mental images). But don't be discouraged, this is mainly due to a lack of practice. You can start with a simpler image. For example you can try visualising a really bright column of violet air emerging from your Crown Chakra at the time of exhalation.
Here is a variation on the previous exercise:
Visualise a shining Lotus flower with a thousand petals, on the top of your head. As you breathe in, visualise a stream of white light rising from the first chakra towards the top of your head via the rear of the body (your back) and say the syllable: "SO" (ss-oo).
Feel this light nourishing the thousand-petal Lotus… which is glowing and unfolding more and more … As you exhale, your breath flows in the form of white light back down to the Root Chakra via the front of your body. Then say the syllable: "HAM".
Take twelve breaths in this way. Imagine that the path of your breath (up the back and down the front of your body) is forming an oval of sparkling light which is encircling you and protecting you, and above which the 1000-petal Lotus is resting. Keep concentrating for a moment on the top of your head, with the Lotus present, then end this visualisation by thanking the Universe.
This exercise is very powerful. Do it whenever you want to stimulate your Crown Chakra.