More things I have learned from the Giant Cozy Sweater knitting:
1. This is not what needle felting is supposed to be.
2. It is always a good idea to check that the decreases you’re using on the third piece match the decreases on the first two pieces. Preferably before you’re seaming them together.
3. Use a yarn (sewing-up) needle of a size appropriate to your work from the outset rather than two long finger-tingling seams later.
4. When knitting a piece to be sewn along the selvages to lengthen sideways-knit sleeves, consider how the direction of the knitting will interact with the seam allowance.
5. Avoid vertical garter seams whenever possible, especially with multicolor yarns.
I know people say it’s easy and neat, but it isn’t if you care at all about, oh, keeping the garter ridges straight. The neatest garter seam I ever managed was when I accidentally turned the back of a slipper into a Mobius loop, which pretty much says it all right there.