I've found that it's easiest to put the belt on the crank sprocket first, then around the idler, then around the p-side sprocket (the one that's poised on top of a cam lobe, just waiting to bite you), then put the 17mm on the d-side sprocket bolt to "adjust it slightly to get the belt teeth to engage the grooves on the sprocket. This is backwards from the FSM, but I find that when I do the spin three times thing after install and initial tensioning. I have gotten it right on the first try once, in about 10 head jobs.

Not responsible for advice not taken...