When you begin a project, the pattern usually will tell you how many strands to use. Most patterns require 2 strands of floss. Each DMC skein comes equipped with floss that is 6-ply (which means it contains 6 strands of floss).
In order to get your two strands of floss to begin your project, you will need to cut an 18-inch strand of floss from your skein and then separate two strands of threads from the remaining four. Take the remainder thread and rewind it into your skein. Now you are ready to thread your needle with your two-stranded floss.
There are some exceptions to using two strands. Some patterns have delicate facial features which require using one strand of floss and stitching over one thread on even weave. Another exception is that sometimes a needlepoint project will require all six strands of floss so that stitch coverage will be complete. In that case, no separation of the threads is required.