In 5 easy steps:
1, The new thread is looped, A being the short end. The longer end B is brought up through the loop.
2, And placed over the short end of the original thread D.
3, The encircling loop C must not be tightened. The new thread is tightened at B.
4, drawing the short end D through the encircling loop which tightens behind it.
5, Both short ends are cut quite close to the knot, which is carefully taken through to the inside of the section as sewing is continued.
Illustrations’ & text are taken from Eric Burdett’s book ‘The Craft of Bookbinding’