After you’ve knit the fronts, your sweater will look like this. It doesn’t look much like a sweater, but once you fold it at the shoulder “seam” at the cast on edge, remove the scrap yarn, and put the pieces on the needles in the right order…
Suddenly it all makes sense! Right now the fronts and back are on my needles, but they’re still completely separate. They’ll get joined together when I cast on for the underarms.
I knit across the left front, and then I cast on stitches using the back loop cast on. I picked that one because it’s easy to do, but any single strand cast on method will work here. You could use the cabled cast on, the knitted cast on, or whatever. We’re going to pick up stitches from this edge later on and you won’t be able to see the CO, so the exact method isn’t an important detail. You just need to get the stitches on your needle.
With my underarm cast on, it’s time to join the back. To join it, all I do is knit into the first stitch on the back and keep going using the same strand of yarn I used to knit the front and cast on the underarms. That’s it. That strand of yarn will connect everything, and although things might be loose and weird at that spot right now, it will get reinforced when we pick up stitches later for the sleeve.
The right front gets joined just like the back did, and now all of the sections of the cardigan are on our needle and joined together. The rest of the body including the waist shaping is knit down from here.