What does 如果做这样，他要看不懂。 mean? Is it grammatically correct? I tried to google translate it, which gave me stupid results.
He is 他, as indicated in your example.
A complement is a word or sentence following a verb, providing additional structure.
A complement can be resultative, as in 做完 (do something, and also finish doing it), directional, as in 放下 (let go of something, and to put it down), and so forth.
A potential complement indicates possibility, as in 看得懂; here 看 is the verb (to see), followed by 得 and an affirmative potential complement 懂. The modifier 得 changes resultative or directional complements to potential ones. 做得完 = he is able to finish what he is doing.
A negative potential complement is therefore the opposite, when a verb followed by 不 plus verb indicates that the action does not result in the desired result: 看不懂. He sees, but he is not able to understand.
The full sentence then: 如果做这样，他要看不懂. 如果 = if; 做 = do; 这样 = this way; 他 = he; 要 = will; 看不懂 = not understand. If (you) do like this, he will not understand. (You is inferred from the context.)
Is it grammatical? It is not pretty, but will be understood. It would be better to negate an affirmative complement, but still better to use 了解 than 看懂. It doesn't sound very native.