You will want to set everything out clearly to ensure you know exactly what you are removing and to know where your new concrete or steel lintel will sit. You will want the new RSJ or concrete or steel lintel to rest on a minimum of 150mm bearings so make provisions for this when marking out. You can use a hammer and bolster to chop back the plaster covering the brick. An angle grinder can be used to make a straight line where the opening will run to or the traditional method of hammer and bolster.