If you are unable to reach your feet you can use a strap this will help you keep your shoulders relaxed and you can feed the strap through your hands as you get further down
Breath in and as you breath out fold from the hips.
Keep the shoulders relaxed and do not round them.
Keep the nect long and straight.
As you move you should be aiming to move forward and down so that the chest is heading towards your knees not your nose.
Do not pull yourself down with arms or strap let the body relax into the pose and adjust as you go down.
For many of you your feet will seem a long way of but regular practice those hamstrings will get longer.