Measure about 4 feet of yarn if you have a box a similar to the size show in this Instructable. An easy trick for measuring if you do not have a tape measure is to use a foot long ruler and wrap it back and forth four times.
Another option is to lace the box while it is still on the skein (that is what yarn bundles are called) and leave enough yarn at the end to make a bow. This is a great method if you do not have a ruler. Just make sure you have enough yarn to tie a bow.
Using the scissors, cut the yarn making sure that you cut straight across. Some yarn can unravel and make a mess if you cut on a slant or have dull scissors that pull rather than cut the yarn.
Knot the yarn
Tie tight knots at both ends of the yarn to prevent unraveling. Try to knot as close to the ends as possible. If your knots are not at the very end, cut off the excess yarn.