I didn't have a chalkline at work, or a straight edge long enough, so I marked out the cuts along the bed floor using a piece of string, and a can of spray paint. Just measure out your start points on either side of the bed, and maybe one in the middle just to check alignment, lay a string from one side of the bed to the other and use something heavy to hold it down (I used to 4lb sledge hammers). Then pull the string tight and line it up, set something heavy on the other side, and spray paint over the top. leaves a nice definitive line.
Then comes the crazy part.... cutting a chunk of the bed off. I'm sure some of you saw my other post, but this is what it's like to cut off a piece of your bed....
notice the piece of pipe on top of the bed rails, I actually welded it there to keep the bed from flexing inward or apart after cutting the sides, it will make it easier to line back up.