1. install the required Microsoft.SqlServer.Types assembly; they are not installed by default, and are available from Microsoft here as "Microsoft® System CLR Types for Microsoft® SQL Server® 2012" - note that there are separate installers for x86 and x64.

2. install Dapper.EntityFramework (or the strong-named equivalent); this could be done via the IDE's "Manage NuGet Packages..." UI, or (at the Package Manager Console):

   install-package Dapper.EntityFramework
   

3. add the required assembly binding redirects; this is because Microsoft ships v11 of the assemblies, but Entity Framework asks for v10; you can add the following to app.config or web.config under the <configuration> element:

   <runtime>
     <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
       <dependentAssembly>
         <assemblyIdentity name="Microsoft.SqlServer.Types" 
             publicKeyToken="89845dcd8080cc91" />
         <bindingRedirect oldVersion="10.0.0.0" newVersion="11.0.0.0" />
       </dependentAssembly>
     </assemblyBinding>
   </runtime>
   

4. tell "dapper" about the new type handlers available, by adding (somewhere in your startup, before it tries using the database):

   Dapper.EntityFramework.Handlers.Register();
   

Once the type handlers are registered, everything should work automatically, and you should be able to use these types as either parameters or return values:

string redmond = "POINT (122.1215 47.6740)";
DbGeography point = DbGeography.PointFromText(redmond,
    DbGeography.DefaultCoordinateSystemId);
DbGeography orig = point.Buffer(20); // create a circle around a point


var fromDb = connection.QuerySingle<DbGeography>(
  "declare @geos table(geo geography); insert @geos(geo) values(@val); select * from @geos",
  new { val = orig });

Console.WriteLine($"Original area: {orig.Area}");
Console.WriteLine($"From DB area: {fromDb.Area}");