Firstly cut a long strip thus can be varied in width to give different effects. Width should also be varied according the thickness of the material with finer fabrics needing to be cut wider to stay in place.
Now cut these strips into short lengths according to depth of pile wanted. Suggest 3” as a start. Just cut a few and try it to see if it gives the effect you want. Cutting straight across will give an even, tailored look while cutting on the slant will give a more shaggy look.
Sharp scissors will do a good job while you are assessing whether you enjoy the craft. The process can be speeded up considerably by using a rotary cutter, ruler and self healing board. Don’t cut too much of any one fabric until you try it – you can’t stick it together again if it’s not right. This sounds obvious but it is the most common error.
For even more variation the straight cut pieces can be shredded. This is best done after they are in place for an extremely shaggy look.
Finally try out a variety of petal shapes to produce a wide range of flowers.