Perhaps you begin with a miming game: “can you guess what I am doing?” After miming the action, you elicit the form, “I am drinking a cup of tea”. This type of activity is generally a very good way to present this new tense.
My problem, however, is the “modelling” part of the lesson. As we were taught in our ESL training, a good lesson should be structured according using the formula “elicit, model and drill”. I can’t help feeling that as I was never taught that the present progressive is formed using the auxiliary, be, with verb plus –ing ending when I was a child, then there’s really no point in teaching it to adult learners of English.
Once you have started trying to explain structure, you will find yourself getting deeper and deeper into meaningless discussions about grammar. Most of your learners will already have some knowledge of English grammar, not all of it correct, from their school years. As their teachers were probably non-native English speakers, the information given was probably at best incomplete, if not totally misleading. Perhaps the word “gerund” may enter the discussion. As an English teaching professional, do you really know what a gerund is? What is the best way of explaining it to your students? If your lesson is on the present progressive tense, then you have let yourself get way off track, for the –ing verb is not a gerund, it’s the present participle.
As I’ve already said, when I was three years old and getting to grips with my mother tongue, English, I had no idea what a gerund or a present participle was. I just used them. So teachers, give your students a healthy amount of active communication, and keep the grammar as a necessary, but not overwhelming, sideline to your lessons.