Usually, surveys are launched by individual assessment managers of the various organizational units. Supserusers may also launch surveys.
Go to the Assessment page, locate the assessment in the list of current ones and click the arrow next to it to expand it. Find the survey which is relevant to your OU (if it’s an org unit assessment) and click the “Launch Survey” button.
Fill in the survey options. Click the help links to get more information, and when you’ve finished with your selections, click the “launch” button.