This knot is often called the double uni. It’s confusing because you can actually double your line and tie a uni knot with a single line to gain better knot strength when tying hooks and lures to your line. So we’re calling this knot the Two Uni Join Knot.
Start by overlapping the tag end of your braid and your fluorocarbon several inches. Now loop your braid and tie a simple uni knot by wrapping your line 4 times inside the loop you created. Pull your braid tag end tight to hold the knot in place as you create a loop with your fluorocarbon along the braided line. Wrap your fluorocarbon around the braid and itself 4 times inside your fluoro loop. Pull the fluorocarbon tag end and snug the knot down. Now pull each main line to slide the two knots together. Pull firmly on each tag end again and then pull firmly on each mainline to finalize the knot.
Different diameter lines may require experimenting with more or less wraps and getting a nice solid connection between the two knots. The nice thing about a uni is you can tie it with as many or as few wraps as you like.