Before looking into avoiding Flight Information Regions (FIRs), Upper Information Regions (UIRs), or specific countries, it is best to review all available resources. One of those resources can be the mapping tool that is accessed by clicking in the map thumbnail on Create Flight Plan page. From there you can select the World Countries to display an overlay with a political map. To display FIRs/UIRs you will need to select “Layers” and then scroll down to the Restrictions subsection where you can opt to display Flight Information Regions. You are able to display both the political map and FIRs at the same time. Note that toggling between the World Hi and World Lo maps will change the FIRs/UIRs shown.
Once you are ready to begin inputting your flight planning preferences scroll down to the Avoid FIR(s) field, just below the Routing section. There, enter the FIRs you would like the Auto-Generated route to avoid. You also have access to the Lookup FIR(s) tool to assist with searches.