NHS England have already contacted most of the patients who require shielding (isolation due to an underlying condition).
These patients should have been informed via letter. However, if you haven’t received a communication but feel you may meet the criteria for shielding you can self-declare via this website :
www.gov.uk/coronavirus-extremely-vulnerable
To help understand if you fall into the ‘high risk’ category please see: Understand the difference between ‘at risk’ and ‘at high risk’