Get yourself a pair of poultry shears. Then you can snip, snip, snip up one side of the back and down the other. I do that whenever I'm butterflying the bird.
i cut straight down either side of the chicken backbone with a chefs knife while the chicken is sitting in the upright "sitting" position on the cutting board.... this is how i was taught in culinary school
I use sharp "kitchen shears", as opposed to "poultry shears", to cut up raw poultry, including snipping out the backbone. I find them handier to work with on raw poultry & only use my poultry shears on cooked birds.
I use kitchen scissors. I just flip the chicken on it's breast and make two parallel cuts on either side of the backbone, close to it.
I have two pairs of kitchen scissors, an old Henckels pair and a newer Wusthof pair. It takes a little strength to cut through the bones. There are also poultry shears that look a bit more like garden shears and may make the job a little easier.