Take in some small objects.You need two or three of each, eg two cups, two necklaces, two watches. You can also use pictures of objects.
Show the students the pairs of objects and say something along the lines of "I've got two necklaces, one is long and the other is short. Which do you prefer?". "I've got two cups, one is red the other's blue. Which do you prefer?" Get the students to answer using "the blue cup", "the long necklace", etc. Do this often enough for the students to understand the exercise.
You can then give a pair of objects to a student and tell him / her to ask you the question. (S)he'll probably need help with this and you might want to write the quetion up for all to see. You should then reply saying "the blue one" or "the short one". Repeat this with other students and other objects, always giving the answer using "the xxx one".
You then show two objects and ask a student the question above to elicit the reply using "the xxx one". Write this phrase up on the board and show how it is used so as not to repeat the noun. Practise pronunciation.
As a whole group, students ask and answer questions.
When you feel students have sufficient grasp of the language, put them into pairs and have them ask and answer questions using a series of pictures you have pre-prepared. Alternatively, if you have enough objects, you can circulate these.
Follow up with a written exercise to practise the structure.