SPI, fast & simple
digitalWrite ( DAC_SS, LOW ); // connects to device's LDAC pin
SPI.transfer ( command_data );
SPI.transfer ( low_data );
digitalWrite ( DAC_SS, LOW );
where the upper 4 bits of command_data are 0111 00xx typical per datasheet page 24, with xx being the upper 2 bits of data
and low_data is the remaining 8 bits of data.