How does an alchemist (or ranger with alchemy skill) mix potions?

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Go here
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That moves me to a web page that gives recipes, i.e. merge lt heal with mod heal to get heavy heal.

But my question is how do I do that? I.e. how do I merge two potions? I cannot find it in the manual.

click on the button in your inv screen that looks like two arrows coming togother. then click on one item, and click that item (which should have a + next to it) on the second.