First, get the passive components with low installation height soldered into their place: the resistors, the diode, the crystal, the capacitors, the USB connector and the buttons.
The resistors and the diode: do not implant them directly on the circuit board: sometimes a conductive path runs beneath them and the contact may cause a malfunction. Insert a thin piece of cardboard between the part and the board which can be pulled out easily after the soldering.
The crystal: do not squeeze the crystal to the board for the same reason.
The 3 capacitors can be laid on the circuit board.
The USB connector can also be laid directly on the board. Only two power pins and the legs of the USB’s body should be soldered in, because this USB is not used for data transmission. Be careful, because the data pins are at close quarters, don't make bad contact!  After soldering try if it is well mounted: Connect a USB device (such as a phone charger), and carefully measure the voltage with a multimeter, e.g. on the pads of the 5th and 14th pin of the microcontroller. It must be 5V with a derivation of ± 5%.
The buttons: Flatten their pins slightly with tweezers, for they fit easily into place.
Now we can proceed further with the active components: the LEDs, the transistors and the microcontroller.
When mounting the LEDs you can use a spacer, a thin cardboard strip for example, for they are at the same height. Right insertion is important because they will not turn on if not aligned properly. The silk screen on the circuit board will give a help: as you can see, the cylinder of the LED is a bit cut off at the bottom. CAUTION! Not all LEDs are made with cut  off on the cathode side!
The painting will also help to plant the 3 transistors to the correct position.
Only the microcontroller remains. After having put it in its place, when it is not soldered yet, gently, you can try to operate the circuit. If you push the microcontroller into the board and you feel that it sits tightly, connect the USB. If you have made it fine, the clock will start. It is possible that not all the pins will be in contact, so not all the LEDs will be lit properly, but if it is flashing every second, it means that you made a good job. I recommend this method only for the experienced because of the possible risk of static discharge, which may damage the microcontroller. When soldering it, be careful and do not solder the 18 pins one after the other but have a few seconds of pause after every 2 or 3 pins, as cooling time for the microcontroller. This way it will not overheat. Of course, you can use sockets, but it is not included in the package.
If all goes well, your clock is now ready. When you connect it to the supply voltage, the clock starts. The initial value is 14 hours 15 minutes 0 seconds. The most practical solution for the power supply is an unused but working USB phone charger.