In miniature work there are two main ways to sew pieces of knitting together, these depend on the location of the seam or the stitch used.
Oversewing is best used to join seams where the knitting is worked in rib or garter stitch:
The needle is taken through both pieces of knitting and over the seams:
The stitches do not show up on the right side of the fabric and leave a very flat seam:
How to work backstitch can be found here
Backstitch is usually used to join side and underarm seams in a flat pattern, such as stocking [stockinette] stitch. Backstitch needs to be worked as close to the edge of the seam as possible or the seams will become bulky.
The needle is brought through the two pieces of knitting and then taken from front to back as shown:
The stitches start to overlap at the back of the work, with the second stitch:
Continue in this way to the end of the seam: