I reckon the easiest way to do it is as if we are teaching a Latin based language.
By now they should know as subject: I, you, he, she, it, we, they
and the object: me, you, him, her, it, us, them
From here, only the phonetic differences are important.
me - my, you - your, he - his, etc
Practise these by rote - the Japanese will love this. Go through it 100 times! Either around the class, or picking out individuals, especially if they
have problems. 100 times me/my, etc - they will find it amusing, especially if you keep the pace going.
Then let them practice their new found words in situation.