This seems like so basic a technique that I can’t find information on it anywhere.
I understand that it means yarn over twice… but what does “twice” mean? Is it make one yarn over, then make another, or is it wrap the yarn twice in one yarn over? Does that make sense?
You wrap the yarn around the needle twice before going to the next stitch. Make sure you know what you are supposed to do with the double YO on the next row. Some patterns call for dropping it to make an extra large loop but some lace patterns call for doing a k1, p1 or p1, k1 in the double YO.