You need to use scissors and clippers designed for cutting hair. Human hair is as strong as copper, and will quickly destroy other blades. Beard trimmers are designed for thick facial hair and won't be effective on your head hair.
Before cutting, wash your hair and comb out any tangles. Wet hair can be cut with scissors, but not clippers.
If you want a simple haircut, electric clippers are the way to go. Clippers will have adjusters or clip-on combs.
These are numbered, with each digit adding 1/8 inch (3cm) to the distance between the blade and your scalp: #2 will give you a buzz cut. Hold the clippers like a pencil, with the wide part of the comb against your scalp.
Cut your hair in sections. Begin by parting down the middle and divide from bottom to top. If you have long hair, you can divide it using hair ties. People with shorter hair can pull pieces of hair taught from the scalp. Bangs are always cut separately, as they'll be shorter than the surrounding hair to stay out of your eyes. If you are having difficulty holding the hair to cut, wet the ends so they stick together.
You can cut normally, or you can open the scissors slightly and slide them along the hair like a razor: This method makes it easier to cut wide pieces evenly. Hair is shorter when dry, so if you're cutting wet hair you should allow for another 1/4" to 1/2" in length.
In the video, she's showing a V cut. This will keep the hair from laying flat, giving it more body. Always cut off damaged hair: unless you've overdone hair coloring, damage will be confined to the ends.
Posted 3962 day ago