It's always a good idea to try to evict a tenant by consent. If they agree then a letter which they sign agreeing to leave will protect you. However, if they refuse to leave, you'll have to file a formal eviction with the court.
You need to know your law, eviction is a very precise process. You issue a demand for rent, notice to quit, and eviction, this needs to go through the court.
a letter is proof of giving someone notice to leave your house. Getting it certified would be further proof. There are certain situations that may prolong the departure even legally such as health of the one being evicted.
