How do you compare two things in French?

First, I would explain that what the student is referring to is known as a comparative. Then, I would explain what a comparative is (an adjective or adverb used in a comparison between two things), and explain how there are three types of comparative (more... than or adjective +er, less... than, as... as). Then, I would ask the student to give me an example of a comparative in English, e.g. Mary is older than Louise. If they give me a 'more ... than' example, I would ask them to give me the other two possible comparatives ('less old', 'as old'). 
Now that the student is clear as to what a comparative is in English, I would give them the French equivalents ('more... than' is 'plus... que', 'less... than' is 'moins... que', and 'as... as' is 'aussi... que'). Then, I would give them example sentences for each type of comparative, pointing out where the adjective needs to come in the sentence (between 'plus' and 'que', 'moins' and 'que', or 'aussi' and que'), e.g. Marie est plus âgée que Louise. Finally, I would remind them that as with normal adjectival constructions, the adjective and the subject have to agree in number and gender, e.g. 'Marie est plus âgée que Louise' -feminine singular, but 'Jean est plus âgé que Marie' -masculine singular, and 'Les filles sont plus âgées que les garçons' -feminine plural. 
Finally, I would ask them to formulate their own comparative clauses, giving them first a few sentences in French with a blank for the comparative, e.g. 'Je suis la plus petite de ma famille. Ma soeur est ... grande que moi'. I would end with giving them sentences in English which they have to translate completely, e.g. 'Paul is as funny as my mum', helping them with the vocabulary if they need, but letting them form the comparative alone. (Only once I had made sure that the student understood the English basics and had properly learned the French equivalents would I teach them the French irregular comparatives ('bon' -> 'meilleur' and 'mauvais' -> 'pire'), and how to form negative comparatives ('pas aussi... que')).