Have a look at the PCB and the included parts to familiarize yourself.
We'll start by soldering low profile components, and then higher profile components.
First, locate all the resistors. There are 4x 4.7kΩ resistors and one 10kΩ. You should be able to tell them apart by the different color stripes. If in doubt, measure with a multimeter.
Resistors are not polarized and can go in any way.
Let us start with the 10kΩ resistor. It goes in R1, on the bottom right of the PCB. Bend the legs of the resistor to point 19 degrees, and pull the legs through. Once the resistor lies flat against the board, bend the legs out slightly on the other side of the board.
Now turn the board over and solder the two legs.
Clip off the excess part with a flat wire clipper.
Next, let us solder the 4.7kΩ resistors. Bend each one and insert into R2 to R5, bending the legs out on the other side. Turn the board over and solder each one, then clip off the legs.
Next out is C1 and C3, the two small small blue ceramic capacitor. Ceramic capacitors are not polarized, and can go in any way.
Push the legs through, and bend on the other side. Turn the board over, solder and clip off the legs.
NOTE: In the first revision of the board, C3 is not marked. It goes in the bottom right side, right above the 10kΩ resistor you soldered in step 1.
Step 4: RTC
Next out is the Real time Clock chip. It has 8 legs, and is marked DS1307. It goes into the footprint marked RTC on the PCB.
Make sure the processor goes in the right way: The half-circle indentation on the chip should match the drawn half-circle on the PCB.
It may be necessary to bend the legs of the chip slightly inward before it will fit in the footprint:
The chip may fall out when you turn the board over to solder. To hold it in place, use a small piece of masking tape. Turn the board over, and solder only one leg first. If you are not happy with the alignment of the chip, re-apply heat to the one leg and then gently move the chip around. Once you are happy with alignment, solder the remaining legs.
Step 5: Crystal
The next step is to place the 32kHz crystal. Direction doesn't matter for this part.
First, bend the legs 90 degrees like so:
Now insert it into the footprint to the right of the RTC chip you just placed. It should be angled slightly inward so that it presses down towards the PCB (but it will work if it points a bit outward too).
Bend the legs, turn the board over, solder the legs and clip off
Step 6: Microcontroller socket
To place the microcontroller, we'll first solder in the 28-pin header. Place it in the long footprint marked MICROCONTROLLER. Make sure that the indentation in one end of the header matches the half-circle in the footprint on the PCB. This part may fall out when you turn the board over, so hold it in place with a clip or a small piece of masking tape.
Solder one pin first, then check alignment. Re-apply heat and gently nudge the header if you are not happy with the alignment. Once you are happy, carefully solder the remaining 27 pins.
Step 7: Battery holder
Before placing the battery holder melt a little bit of solder against the middle pad of the connector marked BATT. This will ensure that the battery gets proper contact. Just a little bit, so that the solder floats evenly across the whole pad is enough.
Now insert the battery holder, it should fit so that the hole points upward, and so that it matches the silk screen on the PCB. If it falls out when you turn the board over, use masking tape.
Turn the board over to solder.
Step 8: USB power connector
Find the USB mini-b connector and insert it to the footprint to the left of the microcontroller. It should fit tightly inside the footprint. Turn over and solder. The pitch for the five pins in the middle is quite small, so be careful not to make any bridges.
PS: The included connector has two pins on the side, but the footprint on the PCB has 4 holes. Solder in the two holes with the pins, and leave the remaining two open.
Next, find C2, the black electrolytic capacitor. It has either 10µF or 47µF written on the side. You'll notice that one leg is longer than the other. This is the positive leg, and it must go into the square hole on the PCB.
Push the capacitor through and pull the legs so that the base sits flat against the PCB.
Bend the legs outward and turn over to solder, then clip off the legs.
Step 10: Temperature Sensor
Find the 3-legged temperature sensor. It goes in the top left of the board, in the footprint marked TEMP.
Orientation is important for this part: Orient it so that the flat area of the sensor matches the flat area of the footprint on the PCB.
Insert, bend the legs and turn the board over to solder and clip off excess leg bits.
Step 11: PTC
Next, place the PTC fuse, the flat yellow disc. It is not polarized and can go in any way. It goes to the left of the C1 and C2 capacitors.
PS: It is a good idea to pull the PCB in a bit tighter than normal (using pinchers), if it stands out too much it might touch the display later on.
Step 12: Slide switch
The slide switch goes to the right. It should be aligned so that the lever points outwards. This part may fall out while soldering, so use a clip or some masking tape before turning the board over to solder
Step 13: Tactile switches
Insert the two angled tactile switches in the footprints marked B1 and B2. They will only fit one way, and should sit quite neatly. Turn the board over and solder.
Step 14: 4-pin male headers
Next we place the two 4-pin male headers that connect to the TWIDisplay board.
These are a bit tricky to keep in place, so use masking tape. Solder only one pin first, and check alignment. It is important that these stand out 90 degrees from the board, otherwise it will be difficult to insert the TWIDisplay later on.
Step 15: 6-pin serial header
There is room for a 6-pin serial header underneath the processor. This is used to attach a FTDI adapter for reprogramming the board. You may skip it for now if you do not plan to reprogram the board. It can always be added later when you need it.
Step 16: Piezo
The piezo buzzer mounts on the back side of the board. Orientation does not matter. It will stick out slightly due to the other pins you soldered on the board. Stick the buzzer in as far as it goes, then bend the legs and turn the board back over to solder.
This is the trickiest part to solder: Be very careful so that you do not burn the plastic of any of the surrounding parts.
Step 17: Battery and Processor
Insert the battery, + pointing up, into the battery holder. The clock will not operate properly without a battery.
Next, place the microcontroller. You may need to bend the legs inward slightly before it will fit in the socket. Use the same technique as you used for the DS1307 chip. Apply even pressure over the chip when pressing it into the socket, make sure that all the legs fit neatly.
Be sure to align the controller so that the half-circle indentation matches the one in the socket (and the PCB).
Step 18: TWIDisplay
Next, locate the TWIDisplay PCB board. The top side, where the two 4-pin female headers go looks like this:
The bottom side, where the display is inserted, looks like this:
Step 19: 4-pin female headers
Let us solder on the two female headers that connect the two boards together. The easiest way to do this is to first insert them into the backpack board, and then insert the TWIDisplay PCB over it like so:
Detach the boards again after soldering.
Step 20: Display
Note the number 1 on the TWIDisplay PCB:
There is a number one on the back of the actual display as well. Align the display so that these two numbers match up, and carefully thread the display through the holes.
Turn the board over to solder. First, solder only one pin and check alignment.
Next, solder the remaining pins, and carefully clip off the excess parts.
Step 21: Final assembly
You are now almost finished. Locate the acrylic enclosure and insert it into the display, the longer part should point down:
There are four screws that go in from the front of the enclosure. Stick the screws in and fasten the bolts on the other side. The nylon screws damage easily, so be careful against over-tightening.
Now insert the display backpack, so that the buttons point upward. The clock will rest against the legs on the backpack PCB.
Finally, insert a USB mini-b cable and plug it into your computer or a 5V regulated USB power supply.
PS: There is no regulator on board, so be careful to only use a regulated 5V power supply. The USB mini-b plug is for power only, and cannot be used for programming the clock.