The sponsors supporting letter is such a personal and individual document it's difficult to find a template.
Everyone has their own story and their own important issues and explanations that need to be highlighted in support of their submitted evidence.
Although not strictly necessary, most people should, in my opinion, write a really good supporting letter.
The letter needs to be put together carefully according to your own personal needs but should ideally outline how and where you met, how long you have been together, the strength of your relationship, how you have maintained contact, important events and meetings between yourselves, the date and location of your marriage and the plans you have made for living in the UK, and why you are opting to live in the UK.
You may also want to very briefly outline how you will support yourselves and if you will be looking for any specific employment etc. which utilises your qualifications.
Try to keep it simple logical and above all, be honest about everything.
If there are exceptional circumstances or anything complicated or unusual about your application or if the sponsor is seeking a concession, it should be explained in the letter.
Many people often make reference to specific items of submitted evidence as a means of trying to strengthen any weaknesses.
Hope this helps.
