^ This. I'm a fan of opiates + benzos but at lower tolerances they aren't worth combining really, often the benzo actually overpowers the opiate feel and takes away from the high if your tolerance to both is low. If you do decide to combine them then just remember that they potentiate each other so if you normally find 60mg oral Morphine to be a good dose, you should use less if you're going to add a 10mg Diazepam (Valium), like 40mg or so.
Plugging is pretty easy, I used to be worried I'd do it wrong somehow when I read up about it too but it's pretty easy, just need an oral syringe, don't even really need to get it in just push the solution in and lay on your side for 10-20 minutes, a lot of people don't even need to lay down afterwards.
Codeine is about 7-10x weaker than Morphine, so 50mg is only like 5-7mg Morphine, it's no surprise it didn't really do anything. 150mg Codeine is a good starter dose to go for if 50mg did nothing (though for anyone else reading the thread who's never tried Codeine, it *is* good to try 50mg first, since some people are hypersensitive).