After finishing the toe, work to the center of the sole, either 1/4 or 3/4 round. Adjust magic loop (or whatever inferior tools you may be using -- I'm looking at you, Knittingbrow) so that this is the beginning of your round, and the midpoint bisects the instep. In this section, you knit the right side of your sock, then knit the left.
For the right side, you'll work some set number of stitches, make a left-leaning decrease, work to your first marker (more about this in a second), make a left-leaning increase, and work to your midpoint. On the left side, work to just before your next marker, make a right-leaning increase, work until some set number before the end of your round, make a right-leaning decrease, and work to end of round.
See? We're decreasing at fixed points so the decreases always stay on the bottom of the sock, but by making increases immediately after the first marker and immediately before the second, we push the line of increases away from the top and down toward the bottom. Here's how I do it:
- Right side: knit to 1 st before center top, place marker, kfb in last st of right side.
Left side: knit 1, place marker, knit to end of round.
Knit 1 round plain.
- Right side: k2, ssk, work to marker, k1, M1L, work to center top.
Left side: work to 1 st before marker, M1R, work to 4 sts before center bottom, k2tog, k2.
- I like the increases to begin from a midpoint, which is why I have that kfb in my set-up round. But there's nothing keeping you from starting your increases at points offset from the center.
- Similarly, your decreases don't have to be 2 sts out from the center bottom. It might reduce some of the foot-hugging qualities of the sock, but I'm making mine 9 stitches out. I'll talk about why I picked this number when we get into the heel.