You can select a minimum of 3 and max of 4 (including a playing captain) or minimum of 4 and max of 5 (including a non playing captain) players 10 days before the draw.
You have to nominate two players to play the singles matches from both teams. The #1 ranked player of each team (according to ATP rankings) plays the #2 of the other team on the first day so there are 2 singles matches on the first day.
There is one doubles match sandwiched between the 2nd and 3rd singles matches. The 3rd and 4th singles matches are played between the #1's of both teams and #2's of both teams so basically all singles players play each other.
Today Nadal (#1) played Monaco (#2) and Ferrer (#2) is playing JMDP (#1).
PS : Ferrer vs JMDP in a crucial 5th set now.