like for mashed potatoes
First, make a roux. Roux is equal parts flour and fat cooked in a saucepan. I usually start with 2 - 3 tablespoons of oil at medium-high heeat, but you can use a vegan butter. Or, the best thing you can do is use the oils from cooking some kind of good vegan meat like a field roast. Then whisk in an equal amount of flour and keep whisking. I like to add a lot of salt and pepper, and cook it until its color is somewhere between milk chocolate and hazelnut. That's it. That's a roux. Super easy, super quick.
Now, get to whisking and add some liquid. A good veggie stock is best, but you can just use water or whatever. Whisk it up good. It's okay if it's kind of thin. Keep it on low heat and it will thicken up.