There’s a good chance you only use one of these words, if you use one at all, and it’s probably envisage. And that’s all you really need. Both can basically mean the same thing: to imagine something, specifically to conjure up a mental image.
There is a slight distinction between the two though, if you want to know exactly how to use them to impress a language nerd.
If you envision something, it’s usually something hypothetical or in the future, and is generally positive:
I envision a world in which people don’t discriminate against each other based on arbitrary criteria.
To envisage something is to picture something more concrete, and not necessarily in the future. We often use it to refer to how something in the present is different from how we’d imagined it:
This meal isn’t what I’d envisaged.
To help separate the two, think of the words they contain.
Envision: we often refer to a dream or fantasy about the future as a vision.
Envisage: visage means face, from the French visage. Envisage actually comes from French, meaning to look something in the face, which is logical as you’re looking at something you know exists and is concrete, so you can easily picture its “face” and look into it.
That makes sense to me anyway, though I can’t envisage a situation in which you might really need to know the difference between the two.