When you need to attach a new ball of two-stranded yarn, stagger the joins. About 4 or 5 stitches before you need to actually join the new yarn, let one strand of your working yarn fall and pick up one strand of the new yarn. Work the 4 or 5 stitches thus (with one strand old, one strand new). Then drop the second old strand and pick up the second new strand. Knit a few stitches, then tug on the ends to tighten. This ensures that the joining of the yarn does not create a single point where a hole in your fabric can form.
