How do you build trust with your partner?

It’s never been that much of an issue. I trust them until there’s a reason not to.

Have sex without a condom and see where he finishes.

Basic every day things. Like if you say you're going to do something then do it. Be reliable and be there when it counts. Don't be late for dates.
Don't be secretive but keep your boundaries.
Do things for them, little things that show you're paying attention and you care - don't forget birthdays etc, make them drinks, ask them if they slept well or if they've eaten...
Be a good partner and a good person basically

Have you always struggled with trust, or is it just in this relationship? If you've always struggled with it, you need to look deeper within yourself to see what triggered it and resolve it; your trust issues will worsen and jeopardise your relationship if not. Speaking from experience.

Be honest and open

Don't get caught cheating. Make eye contact when you lie. And if you have to go to the STD clinic say you caught it from a tractor seat!

With time and transparency. Being honest and considerate. And respect is huge.

I have them hold onto my hand while dangling off a cliff. Works great.