I am doing a program evaluation and want to use difference in difference analysis. I calculated the difference in difference by hand and now want to perform a regression.

I have made a dummy for 1=treatment and 0=no treatment. But since I have done my pre and post test on the same participants (every participant has been measured pre and post) I dont know how to create the pre/post (time) dummy.

I have two columns with pre and post data for every participant, and I dont know how to create the dummy. Help?