Having attended Tanabata and Gion Matsuri so far in Japan (with many more festivals to come) I bought a few yukata.
I started with a set from Takashimaya in Tokyo that cost 8800¥ but came with the yukata, ties, obi, pre made bow and sandals.
Having never worn yukata before we looked up the proper way of wearing it online, J used this guide. The most important rule it seems is that the left side stays on top.
At Gion Matsuri I was able to pick up another yukata at the festival for 1500¥ in a sale.
I think my yukata wearing still needs some improvement, I would like to flatten my chest area some more and get an obiita to keep my obi flat and wrinkle free.
Lastly a picture from behind!