Adding oil to the length of your hair will add moisture to dry ends and also add shine.  It does not take a lot of oil to help your hair, so it might take a little practice to determine the right amount to use that does not leave the hair looking greasy.

If you have not oiled your hair previously, I would recommend doing it the first time at night the day before you plan to wash your hair.  This will give you the opportunity to try out your technique and have a wash planned just in case you used too much oil.

The type of oil used depends on the type of hair you have.  Coarse hair can tend to handle heavier oils such as olive oil.  Fine hair needs a lighter oil such as jojoba oil.  You can experiment with different oils to see what works best for you.  Vegetable/plant oils are preferred.  Mineral oils (such as baby oil) tend to be too heavy to be absorbed by the hair.

The scalp produces sebum, what we commonly refer to as oil.  This oil, when spread down the length of the hair, helps maintain the moisture in the hair.  Moisture is necessary for the hair to remain flexible and have shine.  As the hair gets longer, the natural oils of the scalp are not as able to be distributed down the entire length of the hair.  Using a boar bristle brush can help distribute the oils, if your hair can handle being brushed.  We add oil to our hair to supplement the natural oils.  For this reason we typically do not want to add oil to the hair that touches the scalp.

The hair should be tangle free, freshly combed or brushed before applying oil.  Place a drop or two of oil in the palm of your hand.  Rub your hands together to distribute the oil on your hands. Now flatten your hands out and place your hair between them and slide your hands to the ends.  You should start at about the neck (below the ears).  Continue doing this to cover all of your hair, concentrating on the ends the most.  Once all of the oil is off your hands, you are done! 

Since bedtime oiling is most common, I usually recommend that the hair be either braided or sock rolled after it has been oiled.  This will help the oil stay on the hair (and not your pillow!).

If you are doing a deep treatment with the oil and applied it extra heavy, consider wrapping a damp, warm towel around your head/hair to help the oil penetrate the hair even more.

In the morning you can comb out your hair and marvel at how soft and managable your hair has become!

If your hair seems sticky and stiff, you may have a build up of silicone based products on your hair or a protein build up that has reacted to the oils.  Clarifying your hair of the build up and then repeating the oiling will make a difference in your results.

The frequency of oiling is up to you.  Some people will oil nightly, some only once a week, others just as a deep conditioning treatment.  The condition and dryness of your hair is the best indicator.  I find that during the dry winter months, nightly oiling helps reduce static in my hair and keeps it more manageable.  During the summer, I oil on an "as needed" basis.

If you like, you can also add a small amount of scented oil to your base oil.  Experiment and have fun!