USGS EarthExplorer primer:
If you want to use Bulk Download, you need to install and use "Bulk Order Application" on your desktop (before using step 3b)
(1) Use polygon to mark the vertices of an area of interest.
(2) In the Data Sets tab, select Digital Elevation SRTM, choose SRTM Void Filled (3 Arc-Seconds ∼ 3 MB tile size) or SRTM 1 Arc-Second Global (1 Arc-Second ∼ 25 MB tile size)
(3a) Click Results tab, click the Download Option
, select GeoTIFF, download will start after a few seconds or longer, depending on system load.
or
(3b) Click the Add to Bulk Download
, click View Item Basket (10 Scenes means 10 areas are seclected), click Start Order button, click the square checkbox, select GeoTIFF. Click Submit Product Selection button, click Bulk Order Tracking Page. Launch the desktop "Bulk Order Application" and proceed from there.