The three Carbons are connected to each other and the hydrogens are connected to the carbons. Since carbon makes 4 bonds, it makes a double bond with one of the carbon.
Work Step by Step
The carbons always make four bonds and hydrogens make a single bond. First connect all the carbons with each other with single bonds and hydrogens with the carbons with single bond. Then count if the carbons have four bonds and make a double bond between two carbons to make a double bond.