It's an easy to make a mistake. I will say this much: in English, using a gerund where an infinitive is needed, and vice versa, is wrong, but does NOT make it hard to understand.
Your sentence is incorrect. This is one way to correct it:
To find the area of a circle, multiply the square of its diameter by .785398.
I don't know the formal grammar but I suggest you just practice by reading aloud and repeating other sentences on the same pattern.
--To make good coffee, use one measure of ground coffee per cup.
--To find Arcturus, find the Big Dipper and continue the arc of the handle: "follow the arc to Arcturus."
--To get to Antartica, start anywhere and just head south. You can't miss it!
Here's an example where you'd use the gerund.
--When finding the area of a circle, be sure to use the right formula.