Needed:
Mushroom kit
Thermometer (optional but can be handy - get from a hardware store)
Buy standard plastic storage box with lid, needs to allow at least a few inches space around all sides of the mushroom kit (from B&Q), 
Bag of perlite (B&Q £5 5L from gardening section)

Put about 2 inches deep of perlite in box (about 1/2 bag). Be careful about breathing in the dust (use mask or add some water to the bottom to help stop dusk).  Add water till 1 inch deep in box (about 3 pints). Place mushroom kit (without lid on) in the middle of the box.

Place the box somewhere fairly free from draughts and with some light available (doesn't have to be bright or for very long each day).  Keep box at around mid 70's Farenheit. This will ensure a decent temp to help pinning (formation of mushrooms), and also encourage growth.  

Take the lid off box and use it to gently fan the air in the box, to remove the CO2. Do this at least once a day.

After around 1 week should have fully formed mushrooms. Once the mushrooms cap splits away from the stem then they are ready to pick. You should do this by grasping the mushroom firmly at it's base then twisting.  From this point on the mushroom may grow, but will not be producing any more psylocybin, and therefore just taking nutrients from the cake.

After the first crop the cake will have shrunk down slightly. Turning upside down onto something clean you can take the cake from the box and place directly on the perlite (the same way up as it was originally). This will make the cake soak up moisture and help with further flushes of mushrooms.

When mushrooms have been picked you can either eat them raw, or dry them.  Drying causes some of the potency to be lost, so dry them gently ie in an airing cupboard.  If some of the mushrooms are very big, then slice them in half lenghtways to speed up drying.  Do not put them in the oven  or any other source of heat. Once dry put in airtight plastic bags and store in the freezer.