The double sliding loop is my favorite way to begin a granny square. I find this method fabulous as I can make the beginning ring the size I want.
I worked it with a double ring as I find it more safe and easier to weave in end the tails. And here is a tutorial just to share with you my way to work :
1. Wrap your yarn 3 times around your finger
2. Insert hook into 3 rings
3. Pull the working end of yarn through the rings
4. Work a yarn over with the working end of yarn
6.And pull it through the loop on the hook
8. Work the first round as indicated in the pattern and at the end of round pull the first tail to close the ring
9.Then pull the second tail to close the ring
10. Finally close your round with a slip stitch as indicated in your pattern
Hope you find this helpful
Happy crocheting
Bien à vous
Stefanina