Cthulhu is the Great Old One sleeping on his submerged island, R'lyeh, waiting for the stars to be right, so that he once again can rule the world. Cthulhu has been sleeping for aeons, and will, if necessary, wait until the end of time. The fiction writer H.P. Lovecraft formulated it beautifully in his story, The Call of Cthulhu:
That is not dead which can eternal lie, and with strange aeons even death may die.
The only way we can communicate with Cthulhu these days, is by the way of dreaming. It is mostly one-way communication, as the dream-image of Cthulhu is far to overwhelming in its vastness to open for much spontaneous response from the dream-recipient.
The procedure as such is quite simple, but it takes a while of rigorous training to be able to perform the ritual without major flaws.
The temple should be decorated in suitable colours, such as black, red, deep green and deep blue. If there is a swimming pool in your home, perform the ritual there, and if salt is added (in the water), the results will be even better. If you have a body- or jumpsuit made of green guttaperka, use that for the ritual, or you could paint your body blackish-green. A dark-coloured cloak with a hood, made in a thick fabric will do as a last resort.
The ritual should be done at night, or when you go to sleep (as long as it's dark), and it is of great importance that you are really, really tired; or rather, totally exhausted; both physically, mentally and emotionally. You can surely think of several ways to enter this state.
In this state, before you go to bed, read your favourite Cthulhu-story, and try to imagine, as strongly as possible, the scenes as they unfold before your eyes, BUT DO NOT FALL ASLEEP WHILE YOU ARE DOING THIS! This is _very_ important. It helps to focus on the story. If you should happen to fall asleep, stop reading, go to bed and do the ritual another time.
After you have finished reading, start to whirl counter-clockwise, slowly at first, then faster and faster, while you intone/vibrate/shout/scream/roar:
Do this for 10-15 minutes at first; with practice you can increase it to 30 minutes or even an hour or two. You will of course become very dizzy, but this will diminish with every performance.
When you are done, go directly to bed, but continue to mumble the mantras until you fall into a deep trance-sleep. You will eventually be able to repeat the mantras even after you have fallen asleep.
Sometimes I have found it helpful to make use of a suitable sigil for this ritual, decorating the Temple with it and placing one under the pillow. If you find this useful, I strongly suggest that you design a unique sigil for each and every performance.
After a while of training, hopefully you will be contacted by Great Cthulhu, but don't expect anything for free!