First step is to make a roux from the flour, butter, and salt. Melt the butter in a medium saucepan over medium low heat, then add the flour and salt mixing it well. Let this cook for about 5 minutes. Watch it carefully or it'll burn.
Next add the milk to the saucepan. The milk goes first to drop the temperature so you don't scramble your egg yolks. After the milk, you can add all the other ingredients.
The last step is to stir this mixture constantly with a whisk. It'll take between five and ten minutes to thicken up, but if you stop stirring it'll burn on the bottom or get lumpy. I aim for cake batter consistency, personally.
A little note: This is the first recipe I've uploaded and I'm definitely not a professional cook (pretty sure you could tell that already :). Please let me know how this recipe works out for you if you decide to try it.