Add the remote once, then install apps from it and update them with flatpak update.
flatpak remote-add --if-not-exists frostyard https://frostyard.github.io/flatpak-index/frostyard.flatpakrepo
| App | Architecture | Install |
|---|---|---|
io.github.frostyard.Igloo | x86_64 | flatpak install frostyard io.github.frostyard.Igloo |
Apps use runtimes from Flathub; add it too if you have not. Source: frostyard/flatpak-index.