I have a larger squaring ruler which I use most of the time. Like everyone already stated, squaring is not as easy as lining up one side and whacking off what hangs over.
In the beginning, I marked my size on the ruler with small pieces of painter's tape so I would not line up the wrong measuring line.
A lot depends on the block. If there are any points that need to match up with something else, you want to make sure to line up 1/4" away from the points and square to those. The trick is doing this along all 4 sides.
Don't worry - it all comes easier with practice and the carpenter's saying "measure twice - cut once" applies here too.