Finished knitting the foot of your sock(s)? Great! Now on to creating a gusset…but first a few helpful strategies I’ve developed.
First, the photo above. As soon as my socks are large enough to do so without stretching them, I always stow the yarn cake within each sock when not knitting. This helps immensely in keeping the yarns from tangling.
Next, for this next step of sock construction you will need keep track of the sock top vs. sock bottom (top of foot vs. sole of foot.) I do this by always making sure that the cast on tail is to the right (top photo) — if it is on the left, then I know I am at the halfway point for each sock’s round.
Lastly, you will also need to keep track of knit rounds vs. increase rounds. I do this by utilizing a stitch counter and simple mnemonic devise: I always knit the odd rounds and increase the evens. Why? because ‘knitting is odd.’ 
Ok — now off to the next part of the pattern: Knitting the Gusset
If you’ve knit toe-up socks before, or read alot of toe-up patterns you might be thinking, ‘aren’t we supposed to knitting a heel flap first?’ Normally, yes. But I have developed a method that eliminates knitting a separate heel flap — and has also eliminated the need to pick up stitches along the sides of the heel flap later on.
Ready? Here we go.
Instructions for Increase rounds:
Only to be done on the sock ‘tops’ — not on the stitches that are for the sock sole
M1 (by knit into the front and back of the stitch), K to the last stitch, M1 (by knit into the back and front of the stitch) — repeat for second sock ‘top’
For sock ‘soles’ knit (no increasing during any round)
Did you catch the difference in those to M1 notations?
For the leading one you do a typical Kfb — but for the ending one you Kbf. It is really simple and it gives a better finished line.
So now you follow this pattern:
R1: Knit (because ‘knitting is odd’ )
R2: Increase
As you knit you’ll start to form the above shape. And it will start to form this way on the sides (next 2 photos) — and those are your gussets.
How many increase rows you will need will depend on the foot/sock size, of course. But basically, you want to keep alternating knit and increase rounds until about here:
The rests of the heel will be finished off during the process of ‘turning the heel.’
And please do not let this phrase intimidate you — if you can knit + purl + knit or purl 2 together you can turn a heel. Easy Peasy.
So…there you are. You’ve knit the gusset.
Next post: Turning the heel.
Caution: do not proceed with turning the heel with the stitches allocated on the needles the way they are right now. In the next post I’ll explain how to prep your stitches first.
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Footnote
So, why does this pattern create the gusset by increases and eliminate the step of creating a heel flap? My motivation for developing a different method was the inspiration to find a way to get a heel flap like cuff down (where the heel ‘flap’ is on the back of the heel) instead of the typical flap for toe-up (where the ‘flap’ is under the heel.)
I experimented with this and that sequence, eventually coming up with this one.
I was not looking for an easier way to make toe-up socks – just one that combined the best of both toe-up and cuff-down.
As far as I am concerned this produces a lovely, sturdy and functional sock AND is easier. By eliminating both the heel flap and the need to pick up stitches, I think the socks look more finished. Plus there is no weakness at the pick up line or the very corner, where tiny holes sometimes occur:
Those are perhaps my favorite socks — I wear them all-the-time. That teeny hole doesn’t impact their usefulness, but I will admit it drives me bonkers.
And if you are skeptical that this will all knit up properly, I assure you it will. And it delights me every time it does. I love knitting magic. 
