I soldered a crystal on that board, look for 2 little holes on the board (vias) bewteen the processor and the 2 caps. First apply some solder on the crystal legs and then solder them to the holes.
The major problem is that i had to remove those 2 caps because with the crystal, the board started to hang.
According to this post https://www.ghielectronics.com/community/forum/topic?id=6993&page=3#msg154508 the total capacitance of the two caps should match the capacitance of the crystal. In this case the 2 caps are 14pF and the crystal is 6pF, so something is not right on the documentation… in fact, removing the 2 caps and assuming 5pF for the traces, it matches the capacitance of the crystal…
If you have some, use plenty of flux and a decent iron tip.
After that hard work, i realise that the RTC doesn’t work, it always wake up after 1 minute (see my post https://www.ghielectronics.com/community/forum/topic?id=16978), so good luck and let me know if you make it work.