Sol uses QMK firmware. Documentation on installation, keymaps and compiling can be found here.
You do not need to install bash or compile anything if you plan on using the default keymap.
If QMK Toolbox doesn't work, try these steps
My fork of QMK can be found here
All R1.1 (Group buy) Sol PCBs are pre-flashed with firmware for both halves. If you want to update your keymap, you only need to compile QMK and use QMK Toolbox
To install the default keymap:
Download the left/right eeprom files
Right-click and "Save link as.."
Download the pre-compiled firmware here
Flash the left half
FLIP USB DFU protocol version 1 (doc7618)
Flash: choose your hex file
Plug in the left half and toggle the reset switch
Flash the right half
Flip the #4 (Master) switch to ON
Plug in the right half alone
Follow the same steps as the left, but choose
Flip the #4 (Master) switch to OFF
Plug the leftmost port into your PC, and the two middle ports into each other.
After the initial setup, you can update your keymap using QMK Toolbox, and you only need to flash the master half.
For custom layouts:
Install Bash on Windows
Install and setup QMK and Avrdudess
Edit your keymap as desired and compile hex file
Flash the hex and left/right eeprom to each half (follow guide above)