What you dont need to do is cut into the old wood as this will not respond, i always trim leylandii three times a year, mid may, mid july and early sept, it is always best to keep the bottom of the hedge or if its a single tree broader at the base and slightly tapered up to the top, this ensures light gets to the base of the hedge/tree and so does not become sparse, it also helps shed the snow in the winter, however carefull attention needs to be given to the threat of the conifer aphid, i will post a blog on this, hope this helps.