#detcabbage

That sort of ruler is usually used to cut wallpaper, and one of its side can serve to guide the blade.

So you just place the ruler under the poster you want to cut like this

And proceed to cut following the guide (which I obviously can’t take a picture of since I need both of my hands). You should end up with a nicely cut poster.

If I don’t cut it straight (and that happens to me more often than not), I usually use a paper cutter (?) like the one below to finish it off.