Propose a mechanism for the following transformation.
However, I am unsure of how to begin.
In acidic media, the ester carbonyl will be protonated and the carbonyl carbon attacked by a water molecule. The intermediate will have a proton abstracted by a water molecule and the ethoxy group will be protonated (it can happen either by intermolecular reaction with an H3O+ or by an intramolecular reaction with a proton migration from the protonated intermediate after the water attack). The carbonyl is restored and an ethanol molecule leaves the structure. A subsequent proton abstraction by water forms the carboxylic acid, which will then suffer decarboxylation following the mechanism you presented above. (I'm sorry about my ChemDraw, I suck at drawing even on a computer).