when I do this, I leave the stitches on the needle, cut the yarn with the long tail end. Then thread my yarn needle.
I then start with the stitch on the far end of the needle (on straight needles) - so the 1 stitch of the row you just finished.
then from there through the stitches all the way to the stitch that yarn end comes from.
you now have a loop of yarn from the last stitch you worked, back along your needle, then through the stitches and out the tip of the needle again. Does this description make sense?
once you made this: slip all stitches off the needle, then pull that yarn firmly. It will bring your stitches together nicely and close the top of the hat.
[you CAN go the other way around, too. Take the first stitch to loop through first and then go to the end of your needle. But really, that becomes hard to tighten, sometimes, when the first stitch pulls closed on itself. And it does not close the round as nice. So: Favorably go the way I described]
If your stitches are too tight to make the threading happen, slip them all to a thinner knitting needle first.
or, if you do not have any other needles but still can not fit it, there is a remedy for straight needles (you will not need it on circulars)…
slip all stitches of that last row over to the other needle (your needles go tip to tip, so you slip purlwise). Then you can do the threading from the tip of the needle, because that is where your 1 stitch of the last row now ended up. Tight stitches now can be “picked off” with the tapestry needle and dropped of the knitting needle as they go onto the yarn. therefore you do not have to have 2 needles in the stitches at the same time.
THE THREADING IS DONE!
now you hold it right side out and use mattress stitch. There is a video for that, too.
you sew from the top of the hat down to the brim, weave in the yarn you did the sewing with and also the yarn that you started your knitting with.
Done, wear with pride.