MSBuild custom task “Hello World” walkthrough

Can someone write (or link to) a walkthrough that explains exactly how to create a custom MSBuild task and run it during a build? I'm looking for a custom task that inherits from Microsoft.Build.Utilities.Task and does only this:

public override bool Execute()
{
    Log.LogMessage("Hello world!");
    return true;
}

(I've been working on this for hours and keep getting the "The [whatever] task was not found. Check the following" message. I think I must be missing an essential step somewhere. If there's a clear tutorial I can follow, perhaps I'll figure out where I'm falling short.)