It is highly recommended that after one achieves the second level of Usui Reiki training they consider joining a group that sends out Distant Reiki Healing on a consistent basis. As a Reiki Master Teacher myself, I have my own student based group, and I also have a Facebook group dedicated to that, and am a member of three others, with connections to several more if I need them (for the really serious cases).
Sending Distant Reiki is a wonderful way to help large numbers of people, especially those you can't possible get to you - out of state, in another country, in a hospital, in hospice, in a coma, or during a natural disaster, etc. It also helps one feel like there is really something they as an individual can do to help. we often feel totally helpless in many situations that involve our loved ones or friends many miles or even oceans away. Praying is nice but this is a way you can actually really do something effectively Very empowering!
There are many different ways to do it, some more time and energy consuming than others. There is just sitting down and sending it period, involving using several traditional Reiki symbols or non-traditional ones, or ones used in either Karuna Reiki or other forms of Reiki. This can be VERY time consuming if you have a half a dozen or more people you're sending Reiki to.
Or you can use the Reiki Box method (see the pic's I put up for examples) where you can accommodate as many requests as you can fit into your box. There is also a candle flame technique, a single crystal/stone technique, or a technique involving a crystal grid. There are probably other methods, but these are the ones I know of and do.
I realized recently that you could set up a crystal grid for someone or for a situation and have others send Reiki energy to the grid also. My Distant Healing Facebook group and I are experimenting with this right now.
Different situations call for different ways of sending the energy, so you'll have to weigh each one and see what method would work the best for you and those involved, or the situation.