Now that you've mastered it on the stove...try the microwave - much quicker and tastes exactly the same. I use a glass jug and a whisk - used to always do it on the stove though until I got game. You just can't walk away from it still!
I do a mix that is generally easy to remember that is along the lines of 30g butter, 30g flour, 300mL milk or 60g butter, 60g flour and 600mL milk if you need a larger batch - but I haven't measured out the flour for a while as I just tend to put it in until I get the consistency I'm after.
You just reminded me to get the silverside out of the freezer for tomorrow night - THANK YOU!