First explain that the present continuous is something which is happening NOW – it's real time only. There is also the future use of the present continuous “What are you doing on Monday?” but you might want to leave that for a different lesson.
Mime things so they understand the meaning such as "I am walking to the end of the class". "I am writing on the board".
Bring three students up to the front and tell them to mime an action that they know how to say in English. For example drinking, eating, running, walking, talking, etc. Give the students the action written on a piece of paper if they do not have enough ideas of their own - have those ready in advance.
Tell all three students to start miming their action at the same time - three different actions. Then you say one of them – “He is drinking” - and the class have to call out the name of the student who is drinking. Then say, “She is walking” – and the students name the one who is walking.
Play RELAY RACE
and get the students to pass a sentence down the lines as they mime the action so they practise saying it and they realise that they are doing it NOW.
Then you can give them a worksheet for some writing practise. And get them to swap papers and mark each other's work for reading and reinforcement. Then you want another couple of speaking games to reinforce it.
To contrast the present continuous with the present explain that the present tense is used for something that you do regularly such as a habit. I play tennis every Monday. I never smoke. I always buy diet coke. This contrasts with I am playing tennis NOW. I am smoking now. I am buying coke now.
That might be enough for one lesson and elaborate further at the next lesson. Above all play lots of drill games to get the sentence structures into the minds of the kids.