Basic IL modification

EntryPoint.cs

public class EntryPoint
{
    public static void Main(string[] args)
    {
        // 1
        var assemblyLocation = Assembly.GetExecutingAssembly().Location;
        var pathToOriginalDll =
            Path.GetFullPath(Path.Combine(assemblyLocation, "../path/to/your.dll"));

        // 2
        var pathToNewDll = pathToOriginalDll.Replace(".dll", "AfterWeaver.dll");
        File.Copy(pathToOriginalDll, pathToNewDll, true);

        // 3
        var moduleDefinition = ModuleDefinition.ReadModule(pathToNewDll);

        // 4
        var mainMethod = moduleDefinition
            .GetTypes()
            .Single(t => t.Name.Contains("WhatIWantToWrite"))
            .Methods
            .Single(m => m.Name.Contains("Main"));

        // 5
        var processor = mainMethod.Body.GetILProcessor();
        var consoleWriteLineInstruction = processor.Body.Instructions.Skip(1).First();

        // 6
        processor.Append(processor.Create(OpCodes.Ldstr, "This I Wove!"));
        processor.Append(consoleWriteLineInstruction);

        // 7
        var stream = new FileStream(pathToNewDll, FileMode.OpenOrCreate);
        moduleDefinition.Write(stream);
    }
}