The good news is that you don't have to wade through qualifications to assign an installer - just assign an installation company who will then choose a member of their team. However, if you do know who will be doing this for you, you can search for them using their email address.
To assign an installer to a Job in the 'Preparing' or 'Installation Scheduled' states, select the Job you want to work with in the 'Jobs' section of the Deploy app. Under the section 'Installation Company & Team' click on the 'Choose an Installation Company' button.
A box will pop up that will enable you to select installers in the left-hand pane, clicking on the names will move them to the right-hand pane and assign them to the job.
When you are happy with your selection, click 'Assign'. This will move the job into the 'Installation Scheduled' stage and the Installers you have signed will be notified by email.
Remember that you need to have an Administrator account to access the Deploy app so if you don't have access to this app and need to assign an installer to a job - ask an administrator in your company to do this for you or to add you as an administrator - this article will tell them how to do this.