Download the .lua file and put it in your Extensions directory: https://moneymoney-app.com/extensions/
Get a Trading 212 API key: https://helpcentre.trading212.com/hc/en-us/articles/14584770928157-How-can-I-generate-an-API-key
The key needs all read scopes.
Add an account of type "Trading 212" in MoneyMoney, enter your API key as your username and anything as password. If you have a new enough version of MoneyMoney, it will ask for the API key directly.
Some values might not be congruent to what the mobile app shows you, either due to data staleness or due to weird currency handling. I haven't found a good way to handle this correctly; the T212 API is just weird in this regard. If you have an idea on how to handle this better, PRs are welcome.