Difftastic can be used as Git's external diff or as a difftool. For one-off use run git -c diff.external=difft diff, git -c diff.external=difft show --ext-diff, or git -c diff.external=difft log -p --ext-diff.

For regular use the manual recommends git aliases (examples: ddiff, dshow, dlog, dl, ds, dft) or setting Difftastic as the default by adding

[diff]
    external = difft

to ~/.gitconfig. Use --no-ext-diff to opt out per command.

Alternatively, configure Difftastic as a difftool (the page shows a cmd snippet) — difftool passes extra metadata (permissions, renames) to Difftastic and is safer on older Git versions.

The manual includes gitconfig snippets, recommended aliases, and usage examples.