It is best to plan ahead and find someone to be your assessor before you start working on each section. Take a look at this information on the DofE website about choosing assessors for your Volunteering, Physical, Skills and Residential sections.
The Expedition section of your award is a bit different. Your expedition will need to be assessed by a DofE Expedition Assessor.
For support with finding assessors or if you are changing activity/starting a new section, and need to find an Assessor, take a look at the information on the DofE website here.