Whereabouts on the Kohen Gadol's body was the the Eifod worn?
Rashi and Seforno (on Pasuk 6): The Eifod was worn on the back of the Kohen Gadol, from the waist (corresponding to the elbows) down to the ground. 1
How were the two shoulder straps attached to the Eifod?
Rashi: They were sewn on to it.
Rashbam: Besides being attached to the Eifod, the shoulder straps were also attached to each other, 1 so that between the Eifod, the straps and the Choshen, they covered the entire body of the Kohen Gadol.
Otherwise, in the course of the Avodah, the straps would likely to have slipped off the Kohen Gadol's shoulders, and the Choshen would have fallen (Rashbam).