I think many first-timers have trouble with the double-line (crank sprocket) belt mark. That's where I start. Here's what works for me:

To use the belt marks, I start by wrapping the belt up under the crank sprocket to align the double mark with the 0° tooth (or space--I forget), then jam a special tool (scrap wood) under the sprocket to keep the belt engaged—because it will unwrap away from the 0° reference.

Then I pull the belt up and align the 1st single mark at the Int. sprocket. I keep the belt engaged there with a small, screw-type hose pincher, and do the same when I pull the belt up to align the 2nd single mark at the Cam sprocket.

With the belt secured at all 3 sprockets, both hands are free to work the belt over the tensioner, which I previously squeezed back farther (than the 3mm hole) with big MF waterpumps, holding it there with the locknut.

When the belt is on the tensioner, release the locknut for tension, then tighten the nut and rotate the crank around again to 0° on the Crank sprocket. Make sure the Cam and Crank 0° references still agree—and that the distributor rotor points at the #1 plug wire tower. Ignore the belt marks—they won't line up anymore, as they are ONLY for initial belt positioning.
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Bruce Young
'93 940-NA (current) — 240s (one V8) — 140s — 122s — since '63.