This is from the BC website; hope it helps: 😉
Try to send the letters within 24 hours of the interviews, a maximum of two days later.
Keep it to a brief page. Don't stress over it too much - it's more important just to send something quickly than to delay doing it for days.
When interviewing with several people at one organization, take a few seconds between interviews to jot down some notes about each conversation. Use these notes when writing individualized thank you letters to each interviewer.
When sending letters to several people at an organization, each letter does not need to be completely different from the rest. But don't send identical letters to several people - your letters will generally all end up in your file in the Human Resources department.
Thank the interviewer for his or her time, and reiterate your interest in the job and your enthusiasm for the company.
Whenever possible, say something that will help your interviewer remember you as an individual from amongst all the candidates:
addressing a topic of particular interest (to your interviewer or to you) that arose during the conversation, especially where that topic reflects favorably on your job-related skills (for example, you talked for 15 minutes about the mountaineering trip you led last summer or your research on the health care industry); OR:
addressing an interviewer's specific concern in greater detail than was possible during the actual interview (for example, the interviewer seemed concerned that you did not have the quantitative skills necessary for the job); OR:
re-emphasizing a skill or strength important to that interviewer (for example, the interviewer showed a particular interest in your sports and team-related activities); OR:
inserting a little humor into the process.
Ask for your interviewers' business cards, or write down the interviewers' titles and the proper spelling of their names before leaving the interview site.
Carefully proof read each letter and have one other person do so, too.
Print each letter on a high quality linen or bond paper (available at most copy centers), in white, off-white, cream or light gray, and mail them in matching envelopes.