In Settings › Accounts, choose Sign in to Nexus Mods and approve Mortar in your browser. If that section shows Personal API key instead, copy your key from nexusmods.com (Settings, then API Keys), paste it in and choose Sign in. The key is kept in your system keyring.

Premium accounts download inside Mortar. Free accounts click Mod Manager Download on the Nexus page once per file (Mortar opens each page in turn); enable Handle "Mod Manager Download" links in Settings › Downloads so the button reaches Mortar.

Downloads

The downloads panel opens with Ctrl+J.

A download over 2 GB waits for Continue anyway. Before any download starts, Mortar checks that the file, its unpacked copy and a little spare room fit on the drive; if not, it fails at once and says how much is needed and free. Free some space, then Retry.
Pause keeps what was fetched; Resume continues from there, also after a restart, and replaces Retry after a network error unless the file changed on the server.
Download now moves a download to the front; if every slot is busy, the last running one steps aside and restarts when a slot frees. The arrows reorder waiting downloads.
Settings › Downloads › Download speed limit caps all downloads together in MB/s (0 is no limit).
Pause downloads while the game runs (a game's Play settings, also a profile override) holds downloads while a game you launched from Mortar is open. Downloads you paused yourself stay paused.