How can I explain in an interview that I had a bad relationship with a former boss?

Just don't. You can pull a few details of why a place didn't work out. The second you lay claim that you didn't play nice with your boss, it will plant a seed you don't want planted.

I would simply explain that it wasn’t a good fit. I don’t slam anyone or burn bridges

say you had a difficult relationship; but you learned from it.

be honest- the interviewer should ask about relationships with other bosses

Be objective and logical, and highlight what didn't work and why. Don't bash your old boss. Be pragmatic

You can never say you had a bad relationship with a former boss. I would say we were both growing in different directions.

I would say that my relationship with a former boss was pretty good and that I have gotten along with all my former bosses.

Explain all the issues that led to the bad relationship. Tell them that you wish to move to an organization that values honest input and where you can make a difference.

I wish I could say there is a one-size-fits-all answer here but part of this will require gauging the person interviewing you. The general rule I follow is to avoid issues specific to one person. In the US a prospective employer would not be able to request any details like this from a previous or current employer. So saying anything in that space can do more harm than good. And if we're talking anything but professional - there is no good reason to talk about this in an interview. One way you may be able to say something - disagreement with policies at previous employer instead of an individual. For example, "One thing I did not agree with was a belief that learning anything more to further my skills or knowledge were expected to be done on my own, including any costs." These will help an prospective employer understand what you are looking for in them.

The supervisor applied different standards to different employees when all employees should be subjected to as few standards/expectations as possible.

Unless asked do not address. If asked, had differences of opinion but worked together professionally.

Don't get into it; focus on positive things you learned.