Be very specific as to why you want to join certain lab or certain professor. Write about your publications or any abstracts if you have. Again I personally think it is useless to write about the techniques that you know like PCR and stuff. But if you have publication, write about it and under whom did you work for it. Where did the fundings come from? Your future directions and how your graduate study will favor that.
Try to follow specific pattern, you will find the essay pattern online and fit in these details inside the essays.
Good luck.