Tutorial - making bias strips
A bias strip is a band cut at 45° from grain or cross the grain. It allows the fabric to stretch a little, to curve smoothly and to hang in a better way.
The bias strips are used to neaten edges and reinforce them. There are ready made and pre-folded bias binding at haberdashery but you might make them easily, as follows:
3 steps : cutting, joining, folding
You might also use a special guide / tool that allow to pre-fold the strips.