The trick is to get the belt very taught from the crankshaft, across the idler roller, and onto the intake cam. You want no slack there whatsoever - it normally wants to be a tooth off when you first fit it up. From there it should thread across to the exhaust cam and around the water pump and tensioner with relative ease.

Don't move your cam pulleys to try to tighten things, you will wind up with the cam timing being off.

Are you changing the tensioner as well? If not you need to compress it before you put it back in and you also need to remove the plastic spacer. It compresses very, very, very slowly. Once it is compressed and pinned then you install the bottom bolt, rock it over under the tensioner roller, install the top bolt, and then pull the pin.

After everything is verified the last thing you do is to snap the plastic spacer back on.

...Lee