I always take to validation as understanding that what they are feeling is the truth to them. Focus on the hurt or pain or whatever emotions they are experiencing. Empathise with those feelings and use those feelings to focus your response.
If you are feeling blamed or accused ask additional questions about how they are feeling or how a certain event made them feel. Then go back to empathy before you respond.
Others may have different ways they do it, but this is my method. It helps me to understand how my W feels.
M:34 XW:34 Together: 10y Living: 9y Married: 7y Son:6 Son:4 Separated: 12/28/13 Piecing: 5/2/14 Separated 2nd: 10/16/14 W filed, but pulled it: 11/5/14 papers served: 1/27/15 D final: 3/6/15