An assessment records what you believe about your controls. Testing proves it. Each control your assessments rely on is tested on a regular schedule, using a checklist, and the result feeds back into the next assessment.
Start here
How testing works - the ideas behind it, in two minutes.
Setting it up
The Testing Plan - how often to test, and what counts as a pass.
Control Criticality - which controls matter most.
Test checklists - what a tester checks.