You need 4 stabilizers. They go on Backspace, Enter, Left Shift, and both Space keys. They’re the black things around the key switches you can see in some product pictures.
Have you soldered before? Just take it slow and follow the build guide. I don’t recall any huge pitfalls when I built a similar Keeb.io board a few years ago. The only thing that tripped me up was the Caps Lock key. I was testing the per-key backlighting as I go, and the Caps Lock key uses it’s backlight to show the caps lock status. I re-soldered the LED three times before realizing I just needed to tap the switch to test it.