A breaching sign either symbolizes a crossroads or some other meeting point between the seen and unseen worlds. This is a brief summery of the chapter in Robin Artisson's book An Carow Gwyn which talks about them.
Make sure a window is open if working inside (the wind being able to reach your working space is symbolically important). Place the breaching sign in your ritual space with a lit candle or lantern to the right and a bowl of water to the left. Placing fire and water to either side creates a liminal area for you to work in. Then you put something that symbolizes the spirits or gods you work with behind it. This represents the unseen world.
Go into a light trance (focusing on bodily sensation works well) and say a breaching charm. This is the easiest one:
"Ad regem Eumenidium et reginam eius: Gwynn ap Nudd qui es ultra in silvis pro amore concubine tue permitte me venire domum"
Note: change "me" to "nos" for a group working.
"I Frenin yr Ysbrydion a'i Frenhines: Gwynn ap Nudd, ti sydd draw yn y goedwig Er y gariad at dy gymar Gad i mi ddod mewn i'th gartref."
"To the king of Spirits, and to his queen-- Gwyn ap Nudd, you who are yonder in the forest, for love of your mate, permit me to enter your dwelling."
Use whichever version you like best. Say it three times while gazing at the breaching sign. When you are done, knock on the breaching sign three times with your left hand. You have now finished breaching. Anything you do or say next will have an easier time reaching the attention of the spirits (or gods). You can make offerings by passing whatever you wish to offer over the breaching sign and placing it on the far side (which represents the spirit world). You can make petitions, do devotional work, or acts of sorcery. When you are done, knock on the breaching sign three times with your right hand and the breaching ritual is closed.
If you want to do a breaching rite, but can't light a fire you can substitute a piece of quartz crystal. Drawing symbols for fire and water on either side of the breaching sign isn't quite as effective, but will also get the job done.