I don't envy you the task of making that.
Bechemel from memory was THE final practical exercise at the commercial school I went to. At the joint Navy/AF school I graduate from they didn't even bother teaching it. If you needed bechemel, you poured the powder out of the bag and added hot water, LOL!
/johnny
Bechamel is easy to make. It’s basically milk, butter; flour. I start with bechamel to make cheese sauce for tiny onions or some other cheese thing ... even as a base for tuna casserole.
Sauces that are hard for me are things like Demi-glace which I avoid for the most part.
http://www.epicurious.com/recipes/food/views/white-sauce-or-bechamel-sauce-40046