Shabbat, Chapter Thirteen, Mishnah Two
The first part of the mishnah explains the prohibited labor of making two loops. The second part deals with sewing.
1) He who makes two loops, on either the cross-pieces [nirim] or one the slips [keros], or in a sifter, sieve, or basket, is liable.
2) And he who sews two stitches, and he who tears in order to sew two stitches [is liable].
Section one: We need not fully understand how the ancient loom worked to understand that making two loops, no matter where or how they are done, is considered labor prohibited by the Torah. One is also liable if she makes two loops while weaving a sifter (for sifting flour), a sieve (for separating kernels of grain) or a basket.
Section two: In order to be liable for sewing one needs to sew two stitches. Tearing makes one liable only if the tearing was done with a constructive purpose in mind. If one tears in order to sew up the garment, then one is liable. If the purpose is only destructive, there is no liability.