Populate the microcontroller, resonator, decoupling capacitors, and ICSP header first. You can then burn a bootloader, or burn a sketch directly using the programmer.

The SMD resonator can be tricky to solder by hand, because the pads are entirely underneath the body. You may need to slightly offset the resonator to access the pads with your soldering iron. Make sure all three pads are soldered, with no bridges! If you change the Atmel fuses without the resonator working, you may not be able to recover.

You can leave off the resonator and use the built in clock to reduce part count.

If you want to burn sketches using ICSP, you can use this boards.txt descriptor, taken mainly from the Arduino Pro Mini:

atmega328t.name=ATMega 328 (8 MHz) - USBTinyISP
atmega328t.upload.using=arduino:usbtinyisp
atmega328t.upload.maximum_size=32768
atmega328t.upload.speed=57600

atmega328t.bootloader.low_fuses=0xFF
atmega328t.bootloader.high_fuses=0xDA
atmega328t.bootloader.extended_fuses=0x05
atmega328t.bootloader.path=atmega
atmega328t.bootloader.file=ATmegaBOOT_168_atmega328_pro_8MHz.hex
atmega328t.bootloader.unlock_bits=0x3F
atmega328t.bootloader.lock_bits=0x0F

atmega328t.build.mcu=atmega328p
atmega328t.build.f_cpu=8000000L
atmega328t.build.core=arduino
atmega328t.build.variant=eightanaloginputs

It is useful to have a serial port for debugging sketches. If you want a serial port, populate the serial header along with the 10K pullup resistor and auto-reset capacitor. You might as well run a bootloader if you have a serial port: use this descriptor for boards.txt, and burn a bootloader.

atmega328s.name=ATMega 328 (8 MHz) - Serial
atmega328s.upload.protocol=arduino
atmega328s.upload.maximum_size=30720
atmega328s.upload.speed=57600

atmega328s.bootloader.low_fuses=0xFF
atmega328s.bootloader.high_fuses=0xDA
atmega328s.bootloader.extended_fuses=0x05
atmega328s.bootloader.path=atmega
atmega328s.bootloader.file=ATmegaBOOT_168_atmega328_pro_8MHz.hex
atmega328s.bootloader.unlock_bits=0x3F
atmega328s.bootloader.lock_bits=0x0F

atmega328s.build.mcu=atmega328p
atmega328s.build.f_cpu=8000000L
atmega328s.build.core=arduino
atmega328s.build.variant=eightanaloginputs

For stand alone operation, populate the power section with the LTC3525, inductor, and caps.

WARNING: Only ever power the board from one source at a time! VCC can come from ICSP, the SparkFun serial breakout board, or the batteries. Make sure you do not connect more than one of these at any time.

You may notice in the photos that the VCC pin on my serial header has been removed. I only had a 3.3V serial breakout board and needed 5V for the sensor, so I powered it through ICSP when debugging with the 3.3V serial board.

If repurposing this board for your own projects, make sure of your pin connections before ordering the boards. You may notice an air wire in the photos: I routed the bottom LED originally from A6, which it turns out is not addressable as an output.

If you have a 3D printer, there are STL and DesignSpark Mechanical files for an enclosure. The stand off is sized to self tap with a #4 3/8" screw.