Set the crank before taking off the old belt. Notice the dent in the casting below the toothed crank pulley. When you put the belt on, stick a 10mm socket under it to stop it falling off.
Put the belt on the waterpump followed by the cam pulley and tensioner.
If the anything moves while you tension the belt, it sometimes helps to turn the crank anticlockwise half a tooth or so. This usually mitigates the effect of tensioning the belt.
Remove the socket.
Then do the 2 complete rotations and check the marks line up still.