Often, PCBs come with little bits of the FR4 material poking out of the sides. Simply sand these away with some coarse sand-paper.
(This Sanding block design is by eponra on Thingiverse)
The board has 20 diodes, one for each keyswitch.
First, form the leads of the diodes using the 0.300" position on a lead former. You can use a standard PCB header as an improvised lead former if you don't have one.
(This resistor lead forming tool is by dnewman on thingiverse; the 2nd position on the narrow-short-imperial stl is correct for this project)
Next, Carefully noting the orientation of the diode, insert them in the labeled positions: Match the black strip on the diode body with the strip on the soldermask and insert the dioes into the board. Bend the leads slightly so that the diodes will not fall out of the board.
Then, flip the board over and solder the diodes from the back. Finally, use flush cutters to trim away the excess leads.
Solder two rows of female headers into the two rows of pins at the top of the board. By inserting a Feather board with male headers into the female headers during soldering, you can ensure that the headers are properly aligned, not at a wonky angle. After soldering just one or two positions on each connector, check that they are properly aligned before soldering the rest.
If you are using the 3D printed plate—highly recommended—press the keyswitches into the plate from the back until they click. Carefully note the orientation of each keyswitch; if you will be using the screw-mount case, also note the position of the two screw access holes.
Then, once all the switches are in place in the plate, carefully fit them into the PCB, and solder.
If you are not using the 3D printed plate, fit and solder just one or two switches at a time, starting in the corners. Without the plate, the keyswitch can rotate a surprising amount within the holes; center the leads carefully as rotated keyswitches mean rotated keycaps and the esthetic result is not as good. (The effect was deliberately exaggerated for this photo)