First you need to decide if you're teaching it for expressing intentions (eg I'm just going to make a cup of tea) or prediction (I think it's going to rain.) If you're unsure of the distinction, see this thread.
Then, who are your learners? You'll do it differently if you're teaching 10 year olds or if you're teaching business executives. It will be different if you're teaching a nationality who have a similar structure in their own language (eg French) or if they don't. It will depend on what they've learnt before and so what's likely to help/confuse them- eg have they learnt the present continuous for future arrangements? Have they learnt will for predictions or intentions? If they have, you're going to have to be ready to explain the difference. And how many people are there in the class?
If I were you I'd look at some good text books at the level and see how they do ity. If you're new to teaching it's better to rely on the experience of text-book writers rather than to try and re-invent the wheel every time. However, in case you can't for any reason, here are a few ideas.
In what follows I'm presuming a general purpose adult group of about 12 Ss.
Intentions is usually taught first. You need to find a context where the target language would be naturally used. For example - a group of friends who have just won the lottery and shared the money between them. They're meeting to tell each other what they're going to do with it. They've brought pictures, brochures etc to show each other. The conversation goes along the lines of I'm going to travel round the world. What about the rest of you? What are you going to do Paul?/ Oh, I'm not going to travel, I'm going to open a restaurant.
Present the situation, using the visuals to aid comprehension. Board the target language, and check understanding of meaning (ie the concept of intention) and form. Think about how exactly you'll do this. Notice that the dialogue includes 1st and 2nd person singular, aff, neg and int. - all of them will have to be checked, so that you're sure the learners understand that the basic structure is Be+ going + to + infinitive. By the end this should also be on the board. Do some repetition work of the dialogue (decide if you're going to introduce the weak form gonna
for productive purposes, just for receptive purposes, or not at all), then extend to the third person sing. by asking if they remember eg : What's Paul going to do?
. Check they've noticed the form here too.
Practice : Tell the students they've won £XXXX and give them a handout showing what they could do with it. The handout should have a picture, the necessary phrase, and a price. Eg Pic of a house "buy a house in Tuscany" (price); pic of a plane and the globe "travel round the world" (price). They decide what they're going to do with their money (add a couple of cheaper things so they can do more than one thing if they want) and then mingle asking/telling each other what they're going to do. They try and find other people who are going to do the same, and then sit down together to tell the class :We're going to....
Mix the ss up again and get them to sit in a circle. Each S in turn tries to remember what everyone else is going do do, asking if they don't remember. So eg : Sara is going to travel round the world; Davide is going to buy a house in new York; Antie - I don't remember. What are you going to do Antie? (A : I'm going to open a restaurant) Antie is going to open a restaurant, Hans is ...
etc etc. They each have a go, and the first person to go round the circle remembering correctly what everyone is going to do without having to ask "wins".
Consolidation : You'll then need to find a reading or listening text at the ss level and related to their interests for consolidation - do comprehension work with this - and a final personalisation activity so that they can apply the language to their own experience. What you choose here will, again, depend on who the ss are. With business people for example you might get them to talk about their company's plans for the future : we're going to expand into the Asian market, we're going to increase our product range; we're going to take on more staff.
Gp learners might make some resolutions for improving their English - I'm going to buy some graded readers, I'm going to study for thirty minutes every evening, I'm going to watch some films on DVD