Settings → Account → Log in to Instagram.
Instagram's own login page opens in a separate 520×720 window. You sign in there. The app checks that window's cookies once a second for up to five minutes and, the moment a session appears, writes them to data/ig-cookies.txt: atomically, mode 0600, then closes the window.
The code's own comment: "we never see, store, or transmit the password", and that is accurate: it reads cookies from a finished window and never touches the password field.
The interface says: "Opens Instagram's login page in a window — sign in there (a secondary account is safest). No file needed." The secondary-account advice is sound; keep it.
- Once connected the row reads
Instagram: connected ✓ — log in again. - If the session goes stale, a banner appears: "Instagram needs you to sign in again." with a Reconnect Instagram button. It clears itself as soon as the cookie file is refreshed; no restart needed.
- Disconnect Instagram deletes the file.
- The cookie is included in a full backup, hence the "keep it private" warning.
Cookies are not imported from your system browser. This window is the only path.
Something here wrong, or something missing? Tell us — the manual is corrected against the app, not the other way round.