I forget how to do this...

I want to make an open robe to go over my white drawstring dress for our Georgian picnic that's coming up...I don't remember how to do this. I'm sitting here with fabric and a "i want to make ___" and I'm completely like "um what do I start with? I don't have a pattern. I have zero ideas of what to start with. O.o I wish I knew where the pattern I used for the dress was but I think it's a heavily bastardized version of the simplicity drawstring dress that I made years ago...oh dear apparently sewing isn't like riding a bike.