It's important to take care when stretching your neck because it's quite sensitive. Try drawing an astrix with your neck and head to stretch all the sides of your neck. That's all I know!
I find theres a back stretch that really helps with neck flexibility. Stand with feet shoulder width apart. To test the efficacy of the stretch, turn your neck as far to the left and right as you can, noting how far your vision goes. Now do 10 chest rotations either side, while keeping your hips facing forward. Try turning your neck again and see if your vision goes further round! It works for me every time.