A ojo

[ROBINDESBOIS]

nunca uso un libro de recetas, cocino a ojo ENGLISH?

[Rusty]

cook without a book
cook by instinct
cook with one's senses
cook by taste
cook by eye
cook by eye and taste / taste and eye

throw something together
throw together a bit of this and a bit of that

[poli]

I play it by ear.
or
I just wing it.

[JPablo]

Also,
"at a guess", right?
And,
I cook without measuring anything.

(You also have a ojímetro = at a rough guess)

[chileno]

Right. What my father in law used to call "the famous German precision"

[JPablo]

[Tomisimo]

Another translation for "hacer algo a ojo" is "to eyeball something". It doesn't necessarily work that well with with cooking, but can it certainly can in other contexts.

As for the original sentence:

Nunca uso un libro de recetas, cocino a ojo.
I never use a cookbook, I just make it up as I go along.
I never use a cookbook, I guess.
I never use a cookbook, I just eyeball it.
I never use a cookbook, I use my instinct.

[wafflestomp]

Play it by ear is very common in the US.

[ROBINDESBOIS]

All of them are just wonderful, thank you guys.