If you have had an interview and want to follow up through a letter/email then you want to ensure that you capture what you have learnt from your interview and ensure that you pass this on through your letter.
Make notes on your interview, what you learnt from your interviewers and what you have learnt from your research on the company, the role etc. Then when you are certain you have all the information then you can start to write your letter ensuring to thank them for giving you the time and for giving you a good introduction into their company etc.
If you are looking for an idea of how to write your letter/email please see the below example which you can use as a guide.
Example Follow Up Thank You Letter
Subject: Interview for POSITION and DATE
Thank you for giving me the opportunity to come to your company and discuss the position that you have got available. I found the interview very interesting and informative and the role is certainly something that I feel that I would enjoy doing.
My skills are such that I feel this role would utilise what I have learnt to date and what I can bring to your business.
I was particularly interested in (WHAT DID YOU LEARN FROM THE INTERVIEW? Use this section to note what you leant, what you liked about the role, how you feel that your skills would work in this role, what could you bring to the company etc, use words and phrases that you discussed in your interview as this will show your potential new employers that you were listening and took it all onboard).
If you have any further questions for me, I would be more than happy to try and answer them for you. Please do not hesitate to contact me on (contact numbers, email etc).
Thank you again for seeing me on DATE and I look forward to hearing from you.