All the design files are on the Github page, so you need to install Git and Eagle CAD and run
git clone https://github.com/pepijndevos/accessibletuner.git
The easiest way is to use OSH Park, where you can directly upload the Eagle file. If you want to use another manufacturer, you need to generate gerber files.
Next, wait a few weeks for the PCB's to arrive.
I uploaded a BOM under files, and also a Farnell order with the actual part numbers to save you a lot of time searching.
Note that not every part on the BOM is the exact part I ordered. For the MOSFET's, anything reasonable will work. For the opamp, go for something with high input impedance and rail-to-rail capability, or just take the one I used.
The JFET is a more complicated story. The one I initially used turns out to be EOL. So I'm now using a BSR58. If you want to use something else, you need to verify and adjust the agc_jfet.asc LTSpice model.
There is not much I can say about this. Get your soldering iron (and flux! and wick!) and start soldering. I'd suggest starting with the flattest parts with the most leads, and end with the tallest through-hole components.
The board is Arduino compatible, but to program it, you need an AVR ISP programmer. From the Arduino IDE select the Arduino Uno and your programmer and then click "burn bootloader". This will set the fuses correctly, and also allow you to program it over UART if you want. Alternatively, you can just use "upload using programmer" to upload your sketch directly over the ISP header.