I do this regularly IRL.
First, one must understand the denier.
What views do I & the other have in common?
That determines the starting point for conversation.
I like to point out observations which are very concrete,
eg, lack of ice for polar bears & people who depend upon it.
I find this more powerful than statistics (which they've good reason to distrust).
Once they're open to GW, then I discuss AGW, but not as an advocate.
I simply address how anthropogenic & natural causes would both be at work, & that
the proportion of each is difficult to determine because climate models are immature.
Now they're considering what they once opposed.
It's a start.
Note that I don't claim to understand the GW & AGW debate.
I'm a gearhead...not a climatologist.
I don't have the truth.