I rarely use coconut milk, so it's not something I like to keep around the house. On the rare occasion I do use it, most of it goes to waste because I only need a little bit. How do I make it at home, and can I make a low calorie version?
Most fair sized Asian markets carry boxes of powdered coconut milk/cream. The shelf life on these products is generally quite good, and allow you to make just the amount (and strength) of coconut milk you want by simply adding water.
I don't know about the "low calorie" part of your question but if you take a container of coconut milk and freeze the left over portion in an ice cube tray, and store the ice cubes in a ziplock in your freezer they should last a good long while allowing you to use them in smaller portions in the future.
As for "making your own at home"... first you plant a coconut....
You can make coconut milk at home by using dessicated coconut powder available at stores. Take the coconut powder and make it fine by wet grinding in the mixer (you will be able to extract more milk this way). Take lukewarm water (helps extract the milk better). Add this water to the powder when running though the mixer. (Not too much or the coconut milk will spill over when grinding). Take a wide-mouthed sieve and filter the wet powder through the sieve into a vessel. You can obtain more milk by running the powder through the mixie again with more water. However, this milk will be watery and not as good as the first run.
To make coconut milk, I either use frozen shredded coconut or whole coconut from Indian stores. Take a cup of shredded coconut and add 1/2 cup of lukewarm water (lukewarm water brings out the maximum milk and beautiful white color). Blend it nicely for a minute or two until all the flakes are ground thoroughly. Strain the ground mixture using a thin kitchen cloth colander. We call this extract as "First milk". You can use the residue to make "thin coconut milk" (follow same steps). We call this second extract as "Second milk". I'm pretty sure you cannot extract coconut milk from bakers coconut. In Indian cooking, First milk and Second milk makes difference when you add them in a curry. The order in which they are added also makes difference to a dish.
Well, if you want to make coconut milk from scratch you would first have to get a dried coconut. This is a coconut that's a dark brown colour on the outside. You will know that it's good if when you shake it you hear liquid slushing around inside. Use a hammer to crack open the nut and drain out the liquid (which you can drink). Take a knife to pry the white flesh from the coconut shell and be careful so as not to cut yourself. Chop these into smaller pieces, put in a blender along with some water. Strain and use the liquid as desired.
Here is a recipe using fresh mature coconut to make coconut milk like it is traditionally made. http://ilovetraditionalfoods.blogspot.ca/2013/02/how-to-make-coconut-milk.html
After making coconut milk, don't throw out the pulp because you can make coconut flour out of it. http://ilovetraditionalfoods.blogspot.ca/2013/02/how-to-make-coconut-flour.html