The main thing to remember is that this is a side bend so the body alignment is everything the upper body should not be falling forward.
Front foot faces forwards and the back foot comes in about 40 degrees. The heel of the front foot lines up with the instep of the back foot to create a straight line. Both legs remain straight. Arms out at shoulder height with palms facing the same way that your hips are facing (side of mat). With your in breath side the whole of the upper body forward and as you breath out tip over. You must keep the upper body open it is not all about touching the floor. Watch the upper arm it need to remain in line with the lower one and the hand needs to keep facing the same way as your chest. You can look either at the floor, wall or ceiling depending on your balance and neck. You should be feeling this along upper side and into the hip.