The default design is a 24deg fixed-split, 3mm vertically staggered layout; You can change it at will by modifying the SVG vector design.
The BluePill board has this yellow pin headers that will get in the way, so start by desoldering those:
Now put a solder bridge between the upper pads to make up for the removed jumper caps:
Desolder the reset push button as well, then solder the USB connector (follow the circuit diagram), and finally solder a 1.8k resistor between the A12 and 3.3V pads (I've used a 1k and 0.8k resistors in series).
Once you have the case pieces ready, put the switches in. It's a good idea to hold them in place with some hot glue, so they don't pop out when replacing key caps.
Bend the diodes like this:
And use them to connect the switches in columns:
Solder wires to the bottoms of each column like so:
Now put a 3mm LED in one of the switches, together with a 330 ohm resistor and make sure to isolate it:
Now is just a matter of following the circuit diagram to solder the right wires on the right pads:
You might want to trim the flashing pin header at the bottom of the board to have an easy fit inside the case, but don't get rid of them completely as you might need them in the future if you manage to brick the bootloader when hacking it :)
Use screws to attach the bottom cover; using glue is discouraged as you might need to get inside in the future without destroying the case in the process.
Put some nice key caps on, and you're ready to type!