The safest way to take a hair sample is to ask someone else to help you.
Using your forefinger and thumb, gather 3 or 4 strands and slightly twist the hair together. This will make cutting the hair sample much more accessible and keep all the hair strand in the same direction.
Try to cut the hair sample as close to the scalp as possible, using one cut.
If you do not have anyone to help you, using a mirror may help ensure you avoid injury.