What you need

Windows or macOS, Python 3.10 to 3.12, a recent browser and about 2 GB of free disk space. How to check and install Python.

Computer: Windows or macOS.
Python: Version 3.10, 3.11 or 3.12. Version 3.13 is not supported yet, because one of the add-ons the converter needs (cadquery-ocp) is not available for it.
Browser: One with WebGPU: a recent Chrome, Edge, Firefox or Safari. For the app to open in a window of its own, the launcher looks for Chrome, Edge or Brave.
Disk space: About 2 GB free for the Python environment that is created on the first run.
Internet: Needed once, on the first run, to download the Python add-ons. After that the app runs without a connection.

Check whether you have Python

Open a terminal. On Windows, press the Windows key, type cmd and press Enter. On macOS, open the app called Terminal.
Type the command below and press Enter. On macOS, type python3 instead of python.
Read the answer. Python 3.12.7 means version 3.12 is installed, which is fine. A message that Python was not found means you need to install it.

python --version

Install Python on Windows

There are two ways. The first is the simplest.

Start the app with start.bat (see Start it the first time).
If no Python is found, the window says so and asks whether to install Python 3.12 automatically. Press Y.
Wait while it installs. The launcher then carries on by itself.

Or install it yourself:

Go to https://www.python.org/downloads/ and download Python 3.12.
Run the installer. On its first screen, tick “Add Python to PATH”. This is the step people miss.
Finish the installer, then close any terminal windows that were open, so they pick up the change.

Install Python on macOS

The macOS launcher uses the command python3. If it is missing and you have Homebrew (a tool for installing software on a Mac), the launcher installs Python through Homebrew. Without Homebrew, download Python from https://www.python.org/downloads/ and run the launcher again. Make sure the version is 3.10, 3.11 or 3.12.

Optional: gltfpack

The converter can shrink its GLB files further with Meshopt compression. It does this through a separate free tool called gltfpack, and only when gltfpack is installed and on your PATH. The app works without it. The files are simply larger.