The way it works is that you pick the individual match winners in each tie, and the result of each tie. Example:
USA vs. Serbia:
Troicki d. Isner 3-2
Djokovic d. Querrey 3-0
Bryan/Bryan d. Djokovic/Zimonjic 3-1
Djokovic d. Isner 3-1
Querrey d. Troicki 3-0
Serbia d. USA 3-2
Then, the scores are calculated by country. If one person is competing for their country, their score counts for their country. If there are multiple people per country, then the average score counts.
You could better say USA doubles d. SRB doubles because of the late changes made in DC.
Other than that it sounds good!