It is not clear if you just need an explanation or an equivalent idiom, but I can't think of any right now

by the seat of your pants with little money, without planning, a lick and a promise You can't operate a business by the seat of your pants.

Up, up, and uh oh.

Dear Word Detective: I was just hired to teach a course at the last minute, and when I told a friend about my predicament -- scrambling about to plan in three days a course I've never taught -- she said, "Well, you'll just be flying by the seat of your pants -- it's not like you haven't done it before." I've always assumed that "flying by the seat of one's pants" meant something like "performing as best one can when things must be done quickly and without much preparation." But I started to type the phrase in an e-mail today, and it struck me as just bizarre. What is the origin of this phrase? -- Stephanie Bobo, via the internet.

You are indeed correct that "flying by the seat of one's pants" is not usually considered a restful pastime. The closest experience I've had to your teaching situation is appearing on live radio call-in programs, a harrowing experience which consists of putting myself in the position of taking question after question to which it is highly unlikely I will actually know the answer off the top of my head. Whee!

"To fly by the seat of one's pants" does mean "to do a job the best you can by instinct, training, or experience, without outside aid or instruction," and seems to have been popularized during World War II, though the phrase itself is probably a bit older. It originally meant to fly an airplane either without the aid of instruments (compass, airspeed, altimeter, radio direction finder, etc.) or when instruments were of little or no use in fog or other bad weather. "Flying by the seat of one's pants" in such a situation would mean that the pilot's experience and "feel" for the aircraft (including the actual vibrations, etc. sensed in his seat) would have to substitute for instrument data to guide the aircraft safely. In your case, your experience in previous teaching assignments would, one hopes, be a good guide to your survival in an unfamiliar classroom. If not, my advice is to point out the window, shout "Look! It's that guy from Fear Factor!", and bolt for the door.

to do something by the seat of one's pants - hacer algo guiado por el instinto (Dicc Collins)

This is the equivalent.

We escaped by the seat of our pants =

Logramos escapar por los pelos.

etc.

Bueno...ahora tiene un montón de opciones
Saludos

To quote (paraphrase) from an interview I had with an avionics company in the 1970's – you can't fly a modern (fighter) aircraft by the seat of your pants, you'd shoot it straight out of the sky

by the seat of one's pants: usando la intuición
www.supercable.es/~gweddyn/s.htm

by the seat of one's pants :
using experience and intuition rather than mechanical aids or formal theory, ...
www.m-w.com/cgi-bin/ dictionary?book=Dictionary&va=seat

escape by the seat of ones pants ≈ escape by the skin of one's teeth
= por los pelos
Collins Spanish Dictionary

suerte

a set phrase and it really does mean "por los pelos", not because I say so. It has nothing to do with instinct, regardless of what some dictionaries state. It saddens me when I see that you are going to put a WRONG translation.