Schedule a timeline for your follow up process. A thank you note, whether it be a card or email, should be sent as soon as possible. A phone call to thank the interviewer or point of contact should be conducted no later than a week following the interview.
When composing the note, thank the interviewer for their time and reflect back to the actual interview. Tie the follow up with a conversation you had and reiterate a point in the interview that went well. Take this opportunity to go over anything that you might have missed during the actual interview.
When you follow up, remember that you are always trying to advance to the next process. Be mindful of which phase you are in and try to advance past that. The objective is to review the highlights of the interview and showcase how you stand out from the other candidates.