On the first step, the hydroxy-anion donates an electron pair to the carbon of the carbonyl group, bonds with it, pushes an electron pair of the C=O group to oxygen, and forms a tetrahedral alkoxide.

On the second step, the alkoxide oxygen pushes an electron pair back to the carbon, reinstates the carbonyl group, and pushes away the ethoxide-anion with another electron pair.

On the third step, the carboxylic acid protonates the alkoxide. This step is likely to occur concurrently with the previous step.