For those in the US, it's really quick to send letters to your representatives in support of The Hong Kong Human Rights and Democracy Act of 2019 via this link: https://actionnetwork.org/letters/co-sponsor-hong-kong-human-rights-and-democracy-act-of-2019 (archive)
Vox has some good background on the protests: https://www.vox.com/world/2019/8/18/20810872/hong-kong-protests-beijing-military-police-violence-airport-demonstration-extradition-bill-11-week (archive)
Sunday’s march was organized by the Civil Human Rights Front, the same group that has organized many of the mass protests since they began. The group says that marches will continue until their demands are met, which include a complete retraction of a bill that wold allow Hongkongers to be extradited to mainland China, universal suffrage to elect Hong Kong’s leaders, and amnesty for all arrested protesters.
As tensions rise, a number of US lawmakers have called for bipartisan legislation designed to pressure Hong Kong to align with protesters and to push back against creeping influence from Beijing.
Rep. Jim McGovern (D-MA) and Sen. Marco Rubio (R-FL) reintroduced the Hong Kong Human Rights and Democracy Act earlier this year, a bill that would make it harder for Hong Kong to maintain its special trade status with the US if does not maintain a sufficient degree of autonomy from mainland China.