EpiPens are designed to go through clothing, and should be injected into the outside part of the leg, halfway between the hip and knee. Heart Association recommends keeping the pen in place 10 seconds to allow the medication to auto inject into the patient, and to massage the area for a few seconds afterwards to promote blood flow.
Be aware that EpiPens have a safety on the top of the injector that must be removed before use. Do not touch the orange bottom part of the injector. This is where the needle comes out. The needle is automatically shielded when removed from the leg, so that should not be a major worry, but it needs to be "inserted" into the leg firmly. Follow up after using an EpiPen should be 911, as the patient will need close monitoring for some time. Note the time that the EpiPen was used.