I know how, I just watched a video about this. What kind of decrease do you want? I know how to do right slanting, left slanting or double. Probably though it would be best if you just watch the video I did. Here is a link to it. It is a video by FallingBlox from Ravelry. LINK
All three I mentioned are on the video. If you have dial up connection and don't want to wait hours for it to come in like I did, I could do my best to explain it to you.