First, get some canning jars from the supermarket. I suggest the half-cup size, which are easy to deal with and leave you with many jars to keep or give. Also, make sure you have a stockpot and some kind of rack to hold the jars. A collapsible steamer insert (the kind that looks like a UFO) works fine here. Wash the jars, rings, and lids and set them to dry in a slow oven while you get busy with the pear butter.
Peel a bunch of pears, figuring on about 2 medium pears per half-cup jar. Core them, cut them into large chunks, and drop them in a big saucepan. Add the juice of a lemon or two and a half-cup of sugar. I also like a teaspoon or two of ginger in the mix; do this if it suits your taste. Other people like to add rum or brandy.
Cook until the pears start to come apart, then mash with a potato masher or fork. Taste and see if you want to add more sugar or lemon juice. Continue to cook down until the mixture is a bit thicker than applesauce. I like to leave it a little chunky and rustic, but you can make it beautifully smooth by pureeing it in a blender if you prefer. (Don't puree it straight off the stove unless you like to live dangerously.) Don't stress too much about exact quantities of ingredients or the pear butter's texture. This is low-maintenance stuff, and is meant to be fun.
When the pear butter is cooked down as much as you like, take your jars out of the oven and add the hot pear butter right to them. Make sure the rims aren't sticky and top the jars with lids and rings. Don't make the rings too tight -- you want air to be able to escape during the processing stage.
Process the jars according to the jar manufacturer's instructions. Really, this isn't hard. Botulism is very rare, even with home-canned goods, but you don't want to risk being That Person who gives botulism to all his/her friends. You'd soon have no friends, for a variety of reasons. If you don't have a fancy canning pot and rack, you'll still be fine. Just take your stockpot, put the steamer insert into it, and add the jars, winding a dishtowel around their sides so they won't bang into each other while they're boiling. Fill the stockpot with hot water so that the jar lids are submerged by about an inch, then set the pot to boil for twenty minutes or more. When this is done, turn off the heat and leave the jars in the pan for about five minutes before removing them with tongs. Don't fiddle with the lids for the first 12 hours or so. The lids will ping shut in a cheery manner.
(If you cannot cope with boiling jars, just bottle up your preserves and store them in the fridge once they're cool. They'll keep fine for a few weeks that way.)