For Garter stitch:

First line up your pieces so the cast on/cast off edge of one piece is lined up with the side edge of the other piece.

Thread your sewing needle with a length of yarn, thread your needle from back to front through the first stitch along the cast on/cast off edge.

Take your needle underneath the first horizontal bar along the side edge of your second piece.

Then thread your needle down through the same stitch on your first piece and then back up through the centre of the next stitch.

Take your needle underneath the next horizontal bar along the side edge of your second piece.

Repeat steps 4 and 5 until your seam is complete. You may have to sometimes seam two stitches to one stitch to make sure your seam is even.

For Stocking stitch:

First line up your pieces so the cast on/cast off edge of one piece is lined up with the side edge of the other piece.

Thread your sewing needle with a length of yarn, thread your needle from back to front through the first stitch along the cast on/cast off edge.

Find the horizontal bar between the first and second edge stitches on your second piece, take your needle underneath the first horizontal bar.

Then thread your needle down through the same stitch on your first piece and then back up through the centre of the next stitch.

Find the next horizontal bar between the first and second edge stitches on your second piece, take your needle underneath this horizontal bar.

Repeat steps 4 and 5 until your seam is complete. You may have to sometimes seam two stitches to one stitch to make sure your seam is even.