Union apprenticeship sign-ups near Seattle - what worked for you?

I heard of an apprenticeship with paid on-the-job training for people changing careers. I would rather hear the honest version than the polite one. The application needs a diploma or equivalent, a driver's licence and a short aptitude test. What do you wish you had known?

The aptitude test was mostly maths and reading a diagram. Free practice tests online were enough for me.