If your employer does not provide you with a job duties letter, you can have it signed by one of your colleagues or seniors also.
You can do it on a simple paper only.
- Have your work experience letter from your employer (Which might only include your employment start & finish date, your post on its letterhead)
- Job duties letter (signed by your colleague/senior)
If you need some guidance on format of the same, then please follow these links;
Hi, I worked at a school and my experience certificate was signed by Principal who no longers works. I will go to that school and try to collect a job description letter, is it okay if the job description letter is signed by HR Manager?