Trying to locate a "grid overlay" app/product for painting of a portrait photo taken with Samsung tablet

I am a poor (in capability) artist and want to take a picture of someone & then paint their portrait. I've seen (on TV) that some artists take a picture of their subject & then somehow super-impose a grid on top of the photo thus making it easier to paint the portrait (i.e Portrait Artist of the Year) but I have no idea of how or where to obtain and/or how to overlay the photo. I've searched the Android App stores, etc. but with no luck. Help please.