Use accurate descriptions of activities, deliverables and the professional capacity in which you will work. Include the required employment context, such as purpose, anticipated duration, qualifications and compensation arrangements. The employer should confirm the facts rather than adopt a template simply because another applicant used it.
Test each statement against an identifiable source
For the working draft, note who can verify each point: the supervisor for technical work, human resources for employment terms, and your records for qualifications. This source list is a preparation aid and need not become an attachment. Compare the final description with the offer and any contract so inconsistencies can be resolved.
If confidential work is involved, ask the employer for an accurate explanation at an appropriate level of detail rather than inventing a substitute project. Attach a source to every material statement before the letter is finalized. For duties, the source is usually the supervisor who assigns the work, supported by whatever record exists: a project brief, a work plan, a performance objective, a system the person is responsible for.
For compensation, dates and job title, the source is the employment record. For the qualification relied upon, the source is the applicant's own credential documentation rather than anything the employer holds. Where a statement has no identifiable source, it should not be in the letter.
This discipline also produces a useful by-product, which is a list of the points that are asserted but unverifiable, and that list is what an adviser most needs to see before deciding whether the position supports the classification at all.