Select the large tab on the right side with the ship icon. This takes you to your trade policies screen. Here, you grant trade subsidies to favored trading nations.
Select the amount of the subsidy you wish to provide by clicking on the green circle. Then click on the nation you wish to subsidize. You see a small green circle confirming the subsidy. To cancel the subsidy, click on this icon.
A subsidy provides a percent change to the world price on all commodities traded with that nation. If you are selling, the percent change will reduce the price. If you are buying, the price you pay will increase.
Nations are much more likely to trade with you when they receive these subsidies.
