Aaronsis has already offered such a warm welcome and great support that I feel like my post won't compare!
Can I ask you, is this someone that you've known for a while? Do they know about your mental health in general? I'm asking because I think conversations are quite different depending on your relationship with them already.
For me personally, I like to do tidbits (although that doesn't always work out smoothly!). Saying something like "I'm having a really tough day today" or "Today's a bad day". Is this something you'd feel comfortable sharing or starting with?
Aaronsis mentioned role playing with a counsellor, but if this isn't an option, this is also something you can try beforehand, like practicing what you'd like to say, or writing things down. I know once I wrote a text to my boyfriend who was just in the next room because it felt too tough to say it out loud! Honestly - whatever works for you.
I hope this is somewhat helpful! I don't think that there's an easy answer to this one, but just a bit of trial and error and little steps outside your comfort zone until it's just second nature.