Basically, every three lace rows you are adding enough extra stitches from the center yos that you can split off a half repeat on each side of the corner. There are two versions, but I prefer this one best. There are also two very ruffly ones: corner 2 and corner 2.5. A related corner is Old Shale diagonal increasing corner.
This basically just keeps repeating, with the center of the pattern rows going 6 - 12 - 12 - 18 YOs. I recommend you end on one of the 12YO rows, as pictured.