If you have never written ads before it is more a case of learning from doing.
There are some basic principles when it comes to writing no matter what or why you are writing.
What you do need to write should be clear and concise.
As a new writer the tracking of your ads is imperative as this will show you what is being read and what is not.
You should also do split test runs of your ads and test them in different places. It may be the ad you have written appeals more to one sector of the public than another.
With the actual writing of the ad the title is the most important. If the title does not command attention then anything else you write will probably be irrelevant. (If a title does not appeal to me I won't waste time reading the ad). Once you have a title that works then you need to write some compelling body to the ad, making sure you have something that will encourage the reader to take action by signing up or purchasing whatever you are advertising.
I hope this helps and wish you every success in your SFI business
