You can find the sash for your yukata tacked inside one of the sleeves.

Step 1: Slip on your yukata. Holding the front out ahead of you, align the center back seam with your backbone.

Step 2: Bring the right overlap to the hip bone on your left side. The edge of the fabric should hang straight down vertically inline with you leg.

Step 3: Now bring the left side over the right hip. Again, the edge of the fabric should hang straight down vertically.
Aligning the hemline with the ankles.

Step 4: Using the accompanying sash, tie a bow in the front.

Step 5: If you have a bit of extra length, adjust the over-flap so that it is even. Ideally, the over-flap should come over the waist sash. You can also use this step to adjust any unevenness of hemline.

Step 6: You are now ready to relax in your yukata.