The requirements are easiest to re-member if you recognize that they cover three distinct elements: briefing, notification, and use.
Briefing
Under FAR 91.107(a), the pilot in command is responsible for ensuring "that each person on board the aircraft is briefed on how to fasten and unfasten that person's safety belt and, if installed, shoulder harness." This briefing must be given before takeoff. While it seems unnecessary, practically speaking, to brief a flight examiner during your checkride, you should nevertheless do so. The examiner wants to know that you will comply with the regulations when you have passengers.
Notification
In addition to the briefing, FAR 91.107(b) requires that the pilot in command ensure "that each person on board has been notified to fasten his or her safety belt and, if installed, his or her shoulder harness." This must be done prior to taxi, takeoff, and landing. Note that while the briefing aspect of the regulations applies only to takeoffs, the notification responsibility applies to takeoffs, landings, and movement of the aircraft on the surface. Again, this notification requirement is your responsibility during a checkride, as pilot in command. It's a good idea to get into the habit of asking your passengers to fasten their seat belts as part of your prelanding checklist.
Use
In reviewing the use requirements, we must distinguish between crew-member use and passenger use. The regulation refers to "required flight crewmember" because it is designed to encompass multiple pilot crews. However, for small aircraft certificated for a single pilot, there is usually only one required flight crewmember-the pilot in command of the flight.
FAR 91.105(a) says that each required flight crewmember must keep his or her seat belt fastened during takeoff and landing, and while en route. FAR 91.105(b) says that during take-off and landing, the required flight crewmember must keep his or her shoulder harness fastened, if one is installed, while at the crewmember station, but need not keep it fastened while en route. There is an exception that might apply to the typical single-pilot, small aircraft operation. Because a shoulder harness may interfere with piloting duties, the regulations do not mandate that a required flight crewmember must fasten the shoulder harness if the pilot would be unable to perform required duties with the shoulder harness fastened.
For passengers, FAR 91.107(a) imposes similar use requirements with two exceptions. In addition to takeoffs and landings, passengers must use their seat belts and shoulder harnesses during taxi. However, passengers are not required to use seat belts or shoulder harnesses while en route, although it is probably a good idea for them to do so. (There are also exceptions in the regulations for children under two years of age who may be held in an adult's lap, for children who are secured in an approved child restraint system, and for persons engaging in sport parachuting, who may use the floor as a seat but still need to be belted and harnessed.)
Note that the responsibility for passenger use is not imposed on the pilot. That is, while the pilot in command must brief and notify the passengers on the proper use of seat belts and shoulder harnesses, that pilot is not required to ensure that the passengers use them.
Since briefing and notification on the use of seat belts and shoulder harnesses are requirements placed on you, as the pilot in command, it is probably a good idea to make them a part of your taxi, takeoff, and landing checklists. So, during your training, whether you are flying alone or with an instructor, get into the habit of performing these functions.
