To testify as an expert, a witness needs to be qualified only 'by knowledge, skill, experience, training, or education.' This gives attorneys broad leeway in selecting an expert. First, an attorney should choose a person who practices full time in the relevant field of expertise. Second, the client should be involved in the search for an appropriate expert, since the client will often be involved in the same field or industry. Third, select more experts than are actually needed. After they are selected, experts must be told what to do. This is best done in an engagement letter at the beginning of an expert's involvement in the case. This letter is discoverable by the opponent, so it should be carefully drafted not to suggest what the attorney wants the expert to say on the stand. The first rule in preparing for a trial involving expert witnesses is to know their work thoroughly, including that of opposition expert witnesses. This involves reading their published writings and examining their prior depositions and trial testimony. In preparing their own expert witnesses, attorneys should assist them in explaining complex technical matters simply and convincingly. The structure of the testimony should detail the expert's methodology in arriving at the opinion to be stated in the testimony. A statement of the expert's opinion should be followed by an explanation of the basis or reasons for the opinion. The testimony should be rehearsed with the attorney to make it as clear and convincing as possible.