How to line up the timing marks?

You have the tensioner already released I assume, you're going to have to go back to this step so you can release the tension on the belt, but what you can do to keep the belt where it needs to be while doing one cam at a time, is using a tie wrap, you know the plastic ones that are used for garbage bags, and need to be cut off. You can put a tie wrap around the top of camshaft sprocket to hold the belt in place while you correct the other cams position, you're basically just holding the belt's teeth to the sprocket's teeth in place.
If my 2nd diagram makes any sense of what I'm trying to say, with tie wraps around the intake sprocket and around the belt down on the crankshaft gear, they will hold the belt in place so you're not having to fight with it staying in place while tension is released on it.
Then you should have enough slack to slide the exh cam to the correct place, then tie wrap the exh cam and cut the tie wrap off the intake cam and reposition it, when the int cam is in place, put another tie wrap on it again just to hold the belt in place, make sure your exh cam is the 1/2 tooth below and the belt slack is on the side of the tensioner and follow through with the final steps again.
I would do this so I don't have to try to hold the belt in 3 or 4 places at once, hopefully I'm explaining this good enough, the 3rd pic is the tie wrap I mean, probably not spelling it correctly, but it will help a lot so the belt is not falling off everywhere when adjusting the cams. They only need to be slightly moved, don't rotate them around all the way, just hold the sprocket with a wrench and make your adjustments.
Once you re-tension the belt, make sure all your marks are good, use a straight edge across the cams to verify them. Hope this helps.