There is no special format.
The required facts are these.
full description of responsibilities
start and finish dates
working hours
salary
The description is required because a job title is entirely the employer's choice, and the same words could mean many different things. The oddest job title we had mentioned on the forum was 'assistant to Mr Brown' - we said at the time, Mr Brown could be the company's CEO, or could be the janitor!
Start and finish dates confirm your claim for years of work experience.
Working hours so they can check that this job can be counted as full-time (30 hours per week, and above).
Salary so they can see that this was at the proper rate for a skilled position.
All these points have to be covered by some evidence or other, which can be checked with a third party. It might be four different pieces of evidence, all covering the same job, e.g. job offer letter, HR department job description, contract, bank statements, tax records, pay slips, letter from former colleague, etc. etc.. They don't usually accept a personal affidavit.