this forum is full of answers, you just have to read them.
- Example for Sentinel-1 data: The Order of DEM Creating Steps (you can skip the debursting)
- General discussions on DEM generation: DEM generation
- Tutorial for SAR interferometry with S1 data: http://sentinel1.s3.amazonaws.com/docs/S1TBX%20TOPSAR%20Interferometry%20with%20Sentinel-1%20Tutorial.pdf