Stand straight up with a narrow stance and nearly straight legs. Push your butt backwards, you will start bending at the hips. It’s easier if you keep your arms straight in front of you for balance. Go as low as possible. Note that you shouldn’t actively think about bending forward as in trying to reach the floor. Instead think about pushing your butt back, the bending will happen as an afterthought.
Once you’re fairly comfortable with this stretch you can make it a bit rougher. First thing is to use straight legs but again be careful to not bend your back too much. Second is to point the toes inward, this will target the back off the legs more.