This cape is one of a coolest things I have seen in a while. Such an amazing rain jacket, you get to show off your cute outfit while staying dry! Buy some thick clear plastic sheeting, you could even use a shower curtain like
this one.
Trim it into a circle and cut a hole in the middle for your head, then make a front opening by cutting from the neck hole to the edge along the diameter. (Realise this is confusing haha, I'll add a diagram of this soon). I've added a pattern below, using a basic cape pattern.
Image: Streetstyle Aesthetic