Well, the first thing would be making sure exactly what you need to disclose if you are asked. For most jobs, that means only unspent convictions. If you're not sure, you can get a free copy of your police record here, and Unlock
can help you work out what you need to disclose. After that, it comes down to how you go about disclosure. For a job that has a basic DBS check, you don't have to disclose
unless/until you're asked. When you do disclose, it's probably best to have a self-disclosure statement
that you can hand over at the same time, to support what you're telling them. You can even just hand it over, make a basic statement and invite questions about it. That gives you a chance to add context to the disclosure and explain what was going on for you at the time, especially in terms of your mental health. After that, you can tell them the good stuff about yourself - education, training, qualifications, work, family life, community involvement (including religious worship if applicable to you). Don't be afraid to tell them about your mental health, and make it clear to them that you're now properly medicated and you're putting the past behind you and moving on. Unlike having a criminal record, mental health is a protected characteristic, so they shouldn't discriminate against you because of it.
As Chris Stacey said: Although its not formally part of the sentence that is handed down in court, the criminal record that someone comes away with effectively becomes a second sentence, which can have a long-lasting, if not lifelong, impact.