if you use the whack method you typically hit them right behind the ears, but if you're using another method and something goes wrong, i would just smash wherever i can hit it x: better to have a crushed skull than a rabbit in pain. just a warning it could get bloody that way.
i clean and sell the skulls of my rabbits, so my preferred method is cervical dislocation, where you separate the neck bones, causing instant death. it's a pretty simply method and can be done alone, but requires a bit of coordination. i use a fairly heavy metal rod - doubles as a whacking stick should i need it, but thankfully so far i haven't. with young rabbits you really don't have to pull hard at all, but older adults can require some strength. it's essentially the same as a hopper popper, but it can be pretty much free. make sure you don't have a bad back (which...i do, but i've never had to do more than a few at a time so far.) i always hold my rabbits on the floor until they stop thrashing - it just looks so undignified to hang them up while they're moving, even if they do probably bleed better. it also gives me a little chance to catch my breath. taking a life is never easy, especially if it's one you're attached to
i went take some photos, maybe it'll help. it's just me home so this is from my perspective. i can take some others from third person perspective later when my husband comes home. no bunnies were harmed in these photos! little rosette was a bit upset with me for holding her in that weird position for the few seconds it took to get the last photo, but i gave her kisses and some parsley and she forgave me very quickly.
we have a couple of these metal rods lying around and they make perfect dispatch rods. put bun between feet and lay the rod right behind her ears, in that dip.
lightly put your feet on the rod to hold the rabbit in place. i try to make the transition from this to the next step quick, because this is where they get wiggly and try to run away. grab the rabbit's feet in your dominant hand and...
in one quick motion, put your weight on the rod and yank the feet up at a 90 degree angle. you'll be able to feel the neck pop and the rabbit will start its death throes. you can do an eye test if you aren't sure - if they don't react to you touching their eye, you're good. if you have doubts, you can pull again or use the hammer. it's pretty quick and relatively stress-free for the rabbit. there is some bruising on the neck where it breaks, but we usually don't eat the neck anyway so it's never been an issue.
i envy you guys who can process your rabbits outside, lol. i live too close to my neighbours and the kids play in the streets here all the time and i just know i'm going to end up traumatizing someone's sweet little darling and get yelled at, so i do it all inside in my kitchen sink, heh.