I didn't want to inject in the back of the neck...too many nerves and spine back there to accidentally puncture if the chick would jump. So, I injected into the thigh...subq...that's just under the skin and that worked much better for me. Yes, you must pinch the skin up and make a little tent. Inject into the tent. If you don't pinch the skin, you could inject too deep. You also want to have a moist papertowel that you can spread the fuzz so that you can see what you are doing. Moisten the fuzz so that you can spread it out of the way and it stays there.
Not meaning to be disrespectful but I would not wait until they are 14 days old. You bring stuff into the house all the time on your feet, hair and clothes. I vaccinated my hatched bantams at 1 day old and didn't have any that had any side effects. Ideal hatchery vaccinates at 1 day old. Waiting is just taking the chance that they will be infected before they are vaccinated. Not a chance I want to take.
I use a 25 gauge needle...that's plenty big enough for those small thighs.
Speckledhen...unfortunately, once Marek's hits, whether you chose to euthanize or not, your flock will be carriers. It won't matter how fast you euthanize...symptoms might not appear for 6 - 8 weeks and the virus has been spread through dander for all that time. I don't have a problem with people trying to cure their birds and not just killing them..I have a problem with people who know that their birds are carriers and still sell them to others knowing that they are spreading disease!! No way would I buy chicks from someone who doesn't vaccinate. My trust in others is gone...I trusted the people I purchased chicks from and where did it get me???? Marek's and the loss of everything I wanted out of my chickens...never again!