Buy a t-post (any 3-5 foot rigid metal pole will do but a t-post has perfect geometry). Next, you'll grab your chicken, lay her down on her back or stomach [1], once she has settled down you're going to lay the t-post gently across her spine just a bit behind her skull.
Now place your feet on either side of the t-post to secure it, grab her legs, and in one swift motion pull and stand up.
You will very quickly decapitate the hen and once you do it a couple times it'll be very low stress for both you and the chicken. This latter part is key, if you're stressed, unsure etc, the animals will be the same.
You can improve this further by keeping an upside down traffic cone around, drop the bird into it once slaughtered and that'll contain the flapping/running around you mention.
In my experience, for novices, this is the easiest method for all parties and reduces the risk of slips, mistakes etc.
[1] Back is sometimes easier, they'll often go into a trance. Sometimes you can lay them on their stomach and trace a line to relax them, there are youtube videos of this.