How do you pass a drug test using lasix?

You probably don't.

The basic thought here is undoubtedly something along the lines of "My urine is being tested. Therefore, I want to get the concentration of drugs in my urine down to some level that is within the margin of error of the test; one way to do that is to dilute it, so Lasix - a drug that increases urine production - will help."

The problem is that drug testers are not utter morons, and the resulting pale color of the diluted urine is more or less a dead giveaway that you're trying to hide something, which is just going to get you retested, possibly using a different methodology.

You MIGHT be able to use it to flush out your system if you know far enough in advance of the test... for example, it might help you to pass a test given when you apply for a job, because you have some control over when you apply. It's pretty much useless for random testing once you've GOT the job, though... you'll just need to stay clean.