This is a very complicated and time-intensive approach and requires testing multiple people to identify what parts of your DNA were inherited from which of your 4 grandparents. Testing might include at least 1 parent, 3 siblings, and several cousins.
The value of this approach is that it will potentially eliminate 3 of your 4 grandparental lines and in so doing reduce the number of possible contenders for Common Ancestor to a mere 25%. Taken in conjunction with the other techniques described in this blog, this can considerably narrow down the number of possible candidates for Common Ancestor.
The ISOGG wiki has a summary of phasing and how to do it. It also has some useful references at the end.