So what is a Banishing Ritual? Essentially, Banishing has three aims. The first is that it 'warms you up' as a preparation for doing further ritual, meditation, or whatever, enabling you to put aside 'everyday thoughts' - "what's on TV later", etc. It allows you to take on the role of 'magician' (I'll go into this later on), and to place yourself in the 'centre' of your magical universe - the axis mundi. Secondly, Banishing sets up the space you are working in as 'sacred', so that the loft, bedroom, basement or wherever you are becomes, temporarily, a 'temple' space. Thirdly, a Banishing 'clears' the atmosphere of the area that you're working in of 'psychic debris'.
This made sense to me, so I looked into learning a banish ritual but it seemed that the one I ran across most often was The Hermetic Order of the Golden Dawn's 'Lesser Banishing Ritual of the Pentagram'. I gave it a try a few times but it never seemed to sit well with me. I almost gave up on every using a banishing ritual but Phil Hine's words of advice kept me at it. It was very disheartening using a Banishing Ritual that I didn't like. Then I discovered Peter Carroll's Gnostic Pentagram Ritual and I was saved, finally I had something that I liked, something that felt natural and made me feel good using.