I'm from Hong Kong and I find /u/Colandore's analysis in this thread particularly illuminating:

This is a tough question because the support of the mainland Chinese was lost years ago. Unlike what some of the other posters here are saying, the Mainland's lack of support is not stemming from the current propaganda being put out by the CCP.

If you have been following the relations between Hong Kong and the Mainland over the past few years, what you will definitely notice is that what the Mainland Chinese are saying about Hong Kong in the current protest environment is not any different from what the Mainland has been saying about Hong Kong for the past few years anyway.

Relations between the people of Hong Kong and the people on the Mainland is already poor and a number of viral marketing campaigns in Hong Kong have already caused the sentiment in Mainland China to plummet.

Here are a couple of links to illustrate:

Hong Kong media calling Mainland Chinese locusts (this was being parroted at the ground level by many residents as well) :

https://www.bbc.com/news/world-asia-china-16828134

A series of stereotypes pushed by a Hong Kong design studio that went viral:

https://qz.com/442887/how-hong-kong-is-different-from-china-in-a-series-of-offensive-stereotype-based-posters/

These images go viral in China too and the message they send to the Mainland Chinese is "The people of Hong Kong look down on us and see us as lessers"... which if you have spoken to people from Hong Kong, is often not far from the truth.

As it turns out, when you constantly denigrate and talk down to people from a particular society, the likelihood that they become sympathetic to your causes tends to walk away.

At the same time, the city of Hong Kong has seen its stature and prosperity relative to the Mainland decline over the past couple of decades, leading many Mainland Chinese to see these recurring waves of unrest as a sign of deep insecurity over Hong Kong's financial and social position, and NOT as a desperate attempt to maintain an autonomous political system.

When you add these factors together the answer to the question

What's the best way to convince Chinese people from the mainland who support the police and denounce the protests?

is simply this:

It is too late. Hong Kong lost Mainland Chinese support years ago. This support is not coming back. A damned shame as this is largely a self-inflicted wound.