You just ask them at the front desk on the way in.
Are you talking about the BA lounges at LHR? You most certainly do not need to pre-book showers; indeed, as others have said, there is no way to do so.
AFF Supporters can remove this and all advertisements
My experience there is that you book/request at the reception and they give you a vibrating pager to let you know when your shower slot is ready.