The small capacitors, resistors, button and ferrite cores can go in either way, they don't have a direction. The larger 1000uf capacitors do though. Make sure you put them in correctly (- to the top of the board and + to the bottom, look at the label on the board). Also make sure to check for direction on the DC/DC step down (buck) converters.

While installing the boards that mount on the small pins, I prefer to install them with space underneath the boards. This is just to make sure that if they get hot for some reason, they can cool off. Also, its easier to remove them if for some reason you ever need to. This is personal preference though, so do what you like :)

When connecting the PCM5102 DAC, you only need to connect the 6 pins on the short side of the board, as well as the R and L pins on the long side (right behind the 3.5mm jack).

The resistor networks and the SMD chips have a little dot on them. Make sure that goes to the correct spot on the board (dot always goes to pin 1). Also note that the 2 4.7k resistor networks should face each other!