I found an o-ring, bout the size of your little finger, in my garbage disposal. Did it come from it?

A garbage disposal can be either installed from scratch or instead of an older disposal.

First of all you need to shop for the disposal that best matches your needs.
For example you should consider whether a noisy disposal would be a problem in your household, what kind of plumbing you have, or if you have kids or not.

Here are some guidelines:
- Choose a motor suitable for your household. There are models with lower or higher horsepower, for smaller households with small amounts of food waste, or larger households with more food waste.
- Consider the features that you need. Like a quieter disposal, or one made of stainless steel, or one that connects to the dishwasher. There is a vast variety of options and prices.
- If you have kids you might want to consider getting a batch feed disposal rather than a continuous feed one. The batch feed disposal is a bit more expensive but it is also safer for kids, as it requires the use of a stopper prior operation. While the continuous feed disposal starts running when you flip the switch on and will continue running a bit also after you switch it off. This can be dangerous if someone drops utensils down the drain or sticks a hand inside the disposal.
- If your plumbing connects to a septic tank you need a disposal made for septic tanks.

After shopping for the right disposal, you need to get hold of these tools you'll be needing for the installation:
- Electrical cord
- Disposer wrench
- Hacksaw
- Screwdriver
- Water pump pliers
- Plumber's putty
- Safety supplies: goggles, dust mask, gloves

When you are ready, the first thing to do before installing the new disposal is to turn off power and make sure nobody turns it back on while you work.

Now you can proceed removing the old disposal.
If the old disposal is attached to a dishwasher, loosen and detach the dishwasher hose with a screwdriver.
Then with the wrench unscrew the slip-nut connecting the disposal to the "P" trap waste line, putting a bucket under the trap to catch any waste water sitting at the bottom of the trap.
Remove the old disposal with the screwdriver and take apart the rest of the mounting assembly, including the ring on the sink sleeve, the sink sleeve, the flange, the fiber gasket and the old plumber's putty. Make sure to clean well the sink, the sleeve, the flange and sink opening from old putty and grime before installing the new disposal.

You are now ready to wire your disposal, whether it came with appliance wires attached or if you need to attach them yourself following carefully the manufacturer's instructions (after unscrewing the cover plate beneath the disposal and then screwing it back in).

The next step is installing the drain flange and mounting ring by rolling some plumber's putty between your hands to form a snake about half inch thick and 10 inches long and applying it to the underside of the drain flange. Insert the flange into the drain hole and press down to attach it. You can then snap on the backup ring, fiber gasket and mounting ring.
Use a knife to cut away the extra plumber's putty that will have squeezed out around the flange.

At this point you can place the disposal into the mounting ring making sure that the disposal's outlet is facing the drain pipe connection, and tighten it until it stays in place.
Connect the P-trap, using the hacksaw to cut the discharge pipe to size if too long, and attach it to the outlet of the disposal. Make sure to connect it properly by reading carefully the manufacturer's instructions. If your disposal connects to the dishwasher attach the discharge tube to the disposal as well.

You can now turn the power back on and run cold water into the sink, turning on the disposal to make sure it's working and to check if there are any leaks, and running some food through the disposal to make sure it grinds and disposes properly.

You can also use this video to follow more visual instructions and help you in the process.
http://www.youtube.com/watch?v=DPaMgqnLW1M

Make sure to mantain your garbage disposal in good shape by avoiding grinding certain items like hard shells from shrimps, crabs or other shellfish, unpopped popcorn kernels and hard bones, by avoiding fibrous or starchy foods like banana peels, celery, potato peels, corn cobs, artichokes, fruit pits and hard seeds like avocados or peaches, onion skins and egg shells, as they create clogs and membranes that will ruin the disposal, and by cutting down big pieces of food into smaller pieces.
It is also best to run cold water while the disposal is on, keeping both water and disposer running for about a minute after the sink is cleared. Cold water helps push food down, avoid overheating of the system and unlike hot water, doesn't melt fat, that would re-solidify as a blockage further down the drain.