I can understand why you want to do it at home, I had my cat PTS before Xmas and we did it in the lounge by a nice warm fire, but i must agree with everyone else, do get your Vet to it. They are very practiced at getting veins even collapsed ones.
My cat's veins were almost impossible to find so the vet gave him a sedative first and that relaxed him enough that Andy ( vet) could get his leg vein. The whole process was much more relaxed than taking him to the surgery and my cat was even purring! bless him, even though he was very sick and 17 years old.