Always apply a suitable oil before you start gua sha; preferably care of gerd Deep massage oil and / Arnica Balm
Use "combind" strokes and use the tool untill you can feel heat and you can se a pinkk/ red colour. Sometimes small red dots or bruises can appear and it is not dangerous! It is a sign that you have a lot of stagnation and need to work with your tension. Let the area rest from treatment untill the marks have healed. If you want to avoid brusing try to go light and for shorter time.