The Reiki Symbols
In general, you use a Reiki symbol by drawing it (in the air, on paper or in your mind) and/or saying its name three times. Symbols can be drawn over affected areas or over a client before a treatment. Some Reiki practitioners will not display the symbols or their names to people who have not been attuned. That is why you will sometimes see the symbols referred to by euphemisms such as "the distance symbol" or "the power symbol". I include these for ease of reference and because "the distance symbol" can be easier to say than Hon Sha Ze Sho Nen. The name of each symbol is followed by an approximation of how it is pronounced. The first three (Cho Ku Rei, Hon Sha Ze Sho Nen, and Sei He Ki) are second degree Reiki symbols. The rest are third degree symbols. The Fire Clover is not a traditional Reiki symbol.
Cho Ku Rei (cho (as in blow) koo ray) "The Power Symbol"
Cho Ku Rei is an all purpose symbol. In general, it increases the flow of Reiki or draws lots of Reiki to a particular area. It can be used before any treatment drawn over the body of the client (as can any of the symbols) or it can be drawn over areas in particular need. It can be drawn over food to charge it with Reiki, over medicines, herbs, etc. to increase their effectiveness and decrease side effects, in areas (houses, apartments, cars, etc) to clear away negative energies and as a general protective symbol. The Cho Ku Rei is the symbol that I use the most.
Sei He Ki (say hay key) "The Mental/Emotional Symbol"
Sei He Ki tends to focus on those parts of the energy bodies relating to emotions and mental states. It is very good for healing past traumas (used with Cho Ku Rei and Hon Sha Ze Sho Nen) and works well with positive affirmations. It too can be used in any treatment. Some people use Sei He Ki like Cho Ku Rei to clear areas and protect them.
Hon Sha Ze Sho Nen (hone (as in phone) sha zay show nen) "The Distance Symbol"
Hon Sha Ze Sho Nen is used to send Reiki. You can send Reiki to anyone and anything. It is used to send Reiki over distance and time (such as when using Reiki to heal a past trauma or sending Reiki to a point in the future when you or someone might need it). It is also drawn before sending a distant attunement.
General Notes on Sending Reiki
This is the basic method that I've employed: Begin by drawing the Dai Ko Myo, Hon Sha Ze Sho Nen, Sei He Ki and Cho Ku Rei. Imagine the person you are sending to is shrunk to the size of a doll and situated between your hands. That's about it. Sending Reiki is a effortless as anything else you do with Reiki. After a while you probably won't need to imagine the person between your hands. I usually just sit quietly and name off people that I'm sending Reiki to. You can send to any number of people. The same basic technique is also applied when sending to situations, relationships, to further goals, etc.
Dai Ko Myo (dye koo me oh) "The Master Symbol"
There are actually two Dai Ko Myo's. The Usui or Traditional Dai Ko Myo and the Modern or Tibetan Dai Do Myo. The Tibetan Dai Ko Myo I only use when doing attunements. The Usui Dai Ko Myo I use often. It draws in the most positive energies possible. It is very good to use before any treatment. It can also be used to clear away negativity and as a protective symbol. It can be used when sending Reiki to add energy.
Raku or Fire Serpent (ra koo)
Raku is mainly used when attuning others, it may also be used at the end of a healing session or at other times when additional grounding is needed.