It is really tough. I took a nail and inserted it into the hole on the tensioner arm. Then I used a set of channel locks with a rag around the teeth to pull the arm back away from the chain. When the hole in the arm lines up with the outside edge of the tensioner plate, shove the nail through until it touches the block. The edge of the plate will hold the arm back. Then you have to work the cam sprocket onto the cam. I used a screwdriver to hold the cam forward and tapped the sprocket on with a dead blow hammer. Make sure the crank sprocket isn't seated all the way back as you work the cam sprocket on. Then just work the two sprockets back together until they are both seated.

I've put mine on twice pretty quickly like this. When I tried to install with the original pin in, it would never work. The arm just needs to be pulled back further.

Good luck!