The idler pulley requires a cranked 15mm spanner. The retaining bolt is recessed, and there is not enough room for a socket.
Other than that its pretty straightforwards, but a second pair of hands makes things easier when refitting the belt. The helper should hold the belts over the aircon pulley and power steering pump when you fit the belt over the tensioner.
Its not absolutely nessesary though.
In a clockwise direction when looking at the cambelt end the belt has a staright run between the power steering pump and the aircon pump at the front, it then goes around the idler with the back of the belt running against it, it then goes around the crankshaft pulley, up over the tensioner, then around the alternator pulley and back to the PAS Pump.
You need to bolt the moving tensioner in place first, but don't fully tighten the bolt, then feed the belt over the top pulleys, and then around the others then lever the tensioner out of the way feed the belt over it, then release the tension to tighten the belt. Then tighten the center bolt.
You will see what I mean when you do it. You also need the spanner under the belt when you lever the tensioner out of the way, otherwise it ends up on the wrong side of the belt.