Measure out the flour, butter and milk for your desired sauce thickness and set them close to the stovetop. The process will go very quickly so it should all be at hand.
Preheat the sauce pan over medium heat.
Add the butter to the pan and melt it (but don't brown or burn), then add the flour as soon as it's melted. You can also add both at the same time as I have, just make sure the butter doesn't brown on the edges before melting in the center.
Use the whisk to mix the butter and flour until they are well-combined, and cook the mixture for 3-5 minutes adjusting the temperature as necessary to make sure it doesn't brown. It should begin to smell slightly sweet and nutty. It is okay if it clumps together at this stage, just keep stirring it.
With the whisk in one hand, pour in the milk in a bit at a time (as little as a quarter cup at a time) and whisk it as fast as is safe, being careful not to splash. It may be helpful to have someone assist you at this stage. The bechamel should take on a stringy consistency as shown in the photo.
Continue adding milk while whisking, scraping along the edges of the saucepan to reincorporate any bits sticking to the edges of the pan (these can cause lumps later if not mixed in as you go).
Once all the milk has been poured in, keep cooking for 3-5 minutes while stirring continuously.
Turn off heat and season as you like (optionally ½ teaspoon salt and ¼ teaspoon nutmeg).
Notes
You should have a smooth and consistent sauce. Keep in mind it will continue to thicken as it cools. If the sauce has any lumps or is not your desired consistency, check out the troubleshooting section in the blog post. If you don't plan to use it right away or within 5-10 minutes, cover the sauce with a piece of plastic wrap (stuck directly to the surface of the sauce) or otherwise it will form a skin. If you don't plan to use the sauce within 30 minutes then see the recipe post for making ahead and storing tips.