I followed the instructions for getting Views working with Drupal but it's not working in Backdrop. I see:
SQLSTATE[42S02]: Base table or view not found: 1146 Table 'database_name.civicrm_contact' doesn't exist
Getting Views working with Backdrop is essentially the same as getting Views working with Drupal, although there can be one extra step. So first, follow these instructions to get CiviCRM's tables into Views.
Regarding the possible second step, Backdrop by default uses a simplified structure to specify database credentials (more). This will not cause a problem if you are using the same database for CiviCRM and Backdrop and you do not have a second step.
If, however, you have CiviCRM in a separate database, you need to adjust your Backdrop settings.php in one additional way, changing the default/simplified database string...
$database = 'mysql://user:pass@localhost/database_name';
...to allow multiple databases:
$databases['default']['default'] = array( 'driver' => 'mysql', 'database' => 'database_name', 'username' => 'user', 'password' => 'pass', 'host' => 'localhost', );
@colemanw's note from the Drupal answer applies here, too: "if your [Backdrop] sql user and CiviCRM sql user are different then you need to grant SELECT privileges to the [Backdrop] one for the CiviCRM db."