How to replace alternator belt vectra c diesel?

I have just replaced my alternator on my 2006 vectra It is the 1.9 150 bhp diesel variant.i am not the most proficient DIY mechanic as I now find that I am unable to replace the original belt that was already fitted.it appears to be too short as I can't get it round all the pulleys .when I removed the belt I completely removed the top tensioner in order to remove the belt ,is this where I have gone wrong !!!Any help pls .