Short powerpoint to cover "How OO!/ What a OO!" expressions. A couple slides explaining the expressions and rules for how to use them, a slide with example adjectives and nouns (you can leave this slide up at the end of the presentation during pair work), and a few pictures at the end to practice using the expressions as a class. Then there's a worksheet with 6 additional images to which your students can practice writing. My students worked in pairs. At the end, I picked 1 student per image to go write their sentence on the blackboard.
An important thing for the practice section is that there aren't any 'wrong' answers, and that students might react with different emotions to the same images. Ie, for the sleepy cat picture, your students could use one of countless adjectives to describe this cat. Mine said 'cute/small/sleepy/fluffy/tired\etc' in the 'How OO' and 'What a OO cat!' expressions.
I highlighted the fact that there are so many observations and reactions you might have to a scene or situation, which makes this key sentence really easy to use in everyday English, and to easily communicate your feelings to an English speaker. Hearing this really motivated my students.
On the slide for 'What a OO' in plural form, I point out that if there are many dogs, you could say either 'What a cute dog' or 'What cute dogs' depending on your opinion. If you think only one of them is cute, the prior, and if they're all cute to you, the latter.
Timing wise, I go through the slides in 5-8 minutes, give my students 5-7 minutes to do the worksheet, and 5 minutes to have the selected students write their sentences on the blackboard (students who haven't finished writing can use this time to keep writing, and if you had some students who didn't want to do the activity at all, you can convince them to at least practice writing by copying down one or two of the sentences written on the blackboard). You can wrap it up quickly if your JTE has additional lesson plans, or you can stretch it for another 5 or so minutes by making up scenarios or naming objects on the spot (something my JTE asked, Q: 'What do you think when you see ALT-sensei? A: 'Oh, what a tall teacher!').