Dissolve the bouillon cube in the water before starting. Use beef or chicken depending on the flavor that you want.
Melt the butter in a small sauce pan.
Make a roux by whisking in the flour 1 TBSP at a time. Keep whisking until it is thick and clumpy (it should look just like it does when making gravy).
Whisk in the milk and then the chicken bouillon/water mix.
Add salt & pepper to taste.
The mixture should thicken up as you whisk. Remove it from heat when it has reached the consistency that you want.