If you have created .Net method in some class, compiled it into .dll, and imported it into SQL server as an assembly, you can create user-defined stored procedure that references method in that assembly:
CREATE PROCEDURE dbo.DoSomethng(@input nvarchar(max)) AS EXTERNAL NAME MyLibrary.[Name.Space.ClassName].DoSomething
You need to specify name of the procedure and signature with input parameters that match .Net method. In AS EXTERNAL NAME clause you need to specify assembly name, namespace/class name where this procedure is placed and name of the method in the class that contains the code that will be exposed as procedure.