Put a non sticking pan over low/medium heat, add the margarine and the starch and while the margarine starts to melt, whisk everything until it will look a bit like mashed potatoes
add a third of the milk and whisk pretty fast, so that no clumps form
add the rest of the milk and keep whisking (do not leave the bechamel unattended)
add a pinch of salt (and nutmeg and black pepper, if desired) and mix
taste the sauce and if needed adjust with more salt and spices, according to your taste
keep mixing the bechamel until it gets thicker and creamy (it usually takes around 5 minutes)
when it reaches a texture similar to pastry cream, remove from the stove and cover with a lid if not using immediately (this will prevent a film from forming on top)
if some small clumps have formed it’s ok, if you use the sauce for baked dishes the clumps will melt and you won’t notice them
if bigger clumps form, consider sifting the bechamel through a fine mesh strainer
you can store it in the fridge in a airtight container, after having let the bechamel cool down completely, covered