http://www.dtic.mil/whs/directives/infomgt/forms/eforms/dd0293.pdf3
The major "meat" of the form would be block 6. For the reason of requesting an upgraded discharge, I'd state something along the lines of "The PTSD symptoms the Soldier experienced upon returning from Iraq/Afghanistan resulted in behavioral issues that ultimately lead to the Soldier's discharge" and go into as much detail as you possibly can and attach as much pertinent supporting data as you can including letters from friends and family members noting a change. Be specific as you can on dates or months that changes were noted. Really really really try to develop this as much as you can.
I never filled out this form before, but as a person who filled out plenty of evaluation reports and other forms like this I will tell you that you need to fill up these blocks as completely as possible. Don't provide a single sentence in block 6 - fill it in its entirety if you can.
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I was former military member and I have my own PTSD issues. I really take this personally. I was a junior officer and I saw plenty of guys come back from Iraq and Afghanistan with obvious issues that resulted in a host of behavioral problems that ultimately got them discharged. For the VA to refuse them assistance is a huge disservice.
Please keep us updated. If you and your patient need any help filling out the form, please don't hesitate to ask or PM me. Taking care of Soldiers means a lot to me.