A Reiki treatment consists of the practitioner placing his or her hands on the clients body having only the intent for the energy to flow.  There is no complicated rituals to perform.  

Only the simple exchange of energy between two or more people.  Only the simple intent to share healing.  Only the desire to ease the burdens of another and the willingness to receive.

The Reiki energy is smart for it knows where to go and what to do.  The Reiki practitioner does not direct the energy nor is it necessary to study arcane theories of the universe in order to use it.  It is simply intent which causes the Reiki energy to flow and intent which directs it.  

The best thing for the practitioner to do is nothing, but to get out of the way and allow the energy to do its work.

There is a treatment protocol traditionally taught to Reiki practitioners which involves a series of hand positions.  These positions are well spaced along the clients body and together they provide good coverage of the clients entire body.  

While the energy does go where it is most needed, it is frequently observed that it stays near where the practitioner has placed his or her hands.  

By covering all parts of the body evenly the patient will, by default, get the best treatment possible.  These hand positions are a good place for a practitioner to begin practicing Reiki.  

With experience he or she should feel free to experiment based on the needs of the moment.   In addition to the positions shown for the front of the body, there is a matching set of positions for the back.  Behind the neck, behind the heart, behind the kidneys, and on the sacrum (tailbone).

Any set of hand positions are only a guideline (though I suppose some Reiki Masters insist on using only the hand positions they teach).

Most Reiki practitioners use intuition or other methods to say which positions are to be used in each healing, varying from the traditional positions as the situation warrants.

One common method is the sweep in which the practitioner sweeps his or her hands through the clients energy field looking for hot spots.  These indicate places needing healing energy.

Do this for both legs. You might want to do two positions for each leg. One as shown, the other with one hand on knee and the other on the hip bone. This way the entire energy and nerve system circuitry of the legs have energy run through them.

There is an interesting correlation with Reflexology.  The theory in Reflexology is that when our bodies first form all parts of the body in one place, and as the body grows an energetic connection is formed such that the soles of the feet, the ears, and the palms of the hands, all contain a map of the rest of the body.  

In reflexology the matching places in the feet, ears or hands are massaged to stimulate different parts of the body.  As one becomes more in tune with and aware of the energy pay attention to the energy around the clients feet.  I haven't seen it fail yet that the feet have extra energy at the places matching the major issues in the clients body.

With Reiki it is quite possible to treat oneself as well as others.  Treating oneself is simply placing your own hands on yourself and intending for the energy to run.  

By treating oneself you can more quickly know what the experience of receiving Reiki is like.

Comfort is sometimes harder to arrange depending on where the energy is needed.  Being inventive most of the places in ones body can be reached.  For those body parts which cannot be reached one can try the remote healing techniques to do a remote healing to yourself.

One note about body privacy. Many times the practitioner must place their hands very near (or on) body parts most consider to be private (genitalia). As a system of treating the entire body one certainly would not want to leave such body parts out of treatment.  

At the same time there is matters of privacy to protect.  Would you expect a doctor to not treat those areas simply out of a sense of privacy or embarrassment? No!

Each practitioner has their own way to handle this. They might ask permission before placing their hands in sensitive areas.  Their hands can be placed on top of the clients hands, with the energy being beamed through the clients hands.

The practitioner may also hold their hands above the area in question so there is no touching and beam the energy from a slight distance. The hands in many places very near the sensitive areas trusting that the energy will flow into the areas not directly treated. The remote healing techniques of Reiki level II can be used also.

Practitioner comfort is quite important while practicing Reiki.  A full treatment can easily last for an hour and if the patient is lying on the floor how can the practitioner remain comfortable hunched over for that long?  Better is for the patient to be seated in a chair that gives the practitioner easy access to their whole body.  

Massage tables are very good for Reiki since they can be adjusted, are comfortable, and allows the patient to relax more fully.  Some companies make tables meant specifically for Reiki which allow a roll around chair to go underneath the table.