The command is simple. I just use
dd if=/dev/sdc1 of=/home/myname/imagefile conv=noerror,sync/dev/sdc1 is the partition that I wanted to backup, and after the of= argument, I put the target image file. The noerror option simply made the reading process going even if an error is found, and the sync option.... I'm not sure about it :p. I use it because the tutorials on the web also use it.