For the EPSON TM-T20III, you can get the CUPS driver from [here](https://download.epson-biz.com/modules/pos/index.php?page=soft&scat=32). It's not specific to the printer I've been using, so you can try with other printers.
With that, you can try out your printer and print normal text, images or pdf documents for example. LittlePrynter itself does not require any other software than the ones installed with it, i.e the ones listed in the `requirements.txt` file.
For the Adafruit printer, start by following the guide [here](https://learn.adafruit.com/networked-thermal-printer-using-cups-and-raspberry-pi) to install the CUPS software needed to print images. If you want, you can install it via the command line, [following this guide](https://help.ubuntu.com/lts/serverguide/cups.html).
You can also get some information from [here](https://learn.adafruit.com/mini-thermal-receipt-printer) and [here](https://learn.adafruit.com/instant-camera-using-raspberry-pi-and-thermal-printer) if you're stuck.
The version for the EPSON TM-T20III uses a library called `python-escpos`, which doesn't need a CUPS driver or anything else. It's included in the Python dependencies.
The project only supports the EPSON printer, but you can try to adapt it for other printers using the `adafruit-thermal` branch, but I won't offer support for it.
You should see a folder named `configuration`. Enter it, and duplicate the file named `config.toml.sample`, and rename the copy to `config.toml`. Now, edit this file by following the comments in the file itself.
This command should start a web server with which you can test your configuration. If you plan on exposing your printer to the Internet, and give it an IP / URL, _please, please, please_, don't run it this way. Look at Flask's documentation and read about running a production server. It's a little more work, but it will prevent your computer/server being hacked in too easily.
