I am in total agreement with everyone:
"You don't want to do this yourself, the most peaceful way for her to go is in the hands of a professional, not yours."

Find a vet that will come to your house if you can
not go to the vets office.
I now someone who lives in another state.
She is a vet tech.
2 years ago, She thought she would do a favor
for a freind who requested her dog to be put
to sleep at home. (with-out a vet)
She brought home the syrum and gave old dog
a shot. Something went wrong.

Do not do this yourself, and have
a vet do it for you. My friend still has visions on
what happen and so does the dogs owner.