Offering your internship at this institution of choice
The easiest way of getting an assurance letter from your institution of choice as a prospective national service personnel is by offering your internship at this institution of choice, prior to or during your final year.
Upon completion of your internship request for an assurance letter. If you were punctual and hardworking, they would see no reason not to give you. You should be making enquiries on how to get an assurance letter earlier in your internship days. You should be connecting with staff and relating well with them.
Know someone at your institution of choice
The second way is to know someone who knows someone. This is Ghana for that matter. Whom you know and who knows you is everything. A brother or an uncle might be in good relations with someone at your institution of choice. They could just contact them and see you through.
Contribute towards a developmental project or event in the institution
The third option is to contribute towards a developmental project or event in the institution. If it is a senior high school, you can give a token towards a project they unveil at a speech and price giving day or PTA meeting and use that as a reference point when requesting for the assurance letter.
This is obviously not the best option but letting a known staff stand in for you would be easier. He could just say you just like the school and would want to work with them
It is good to know that you must go in person to request for the letter.
Since you would need to send it as soon as possible and you get reply as of whether they are giving you or not instantly. If you are asked to put it in writing still present it by hand. Posting it and receiving reply through the posting is a long process.