2-spanner job (both in terms of tools required and Haynes rating!)
Jack up / support f/o/s of car, remove road wheel
Remove plastic panel of inner wing to access crank pulley
Note how the belt runs around the various pulleys.
Fit 15mm spanner to the aux tensioner, and turn anti-clockwise to slacken belt. Remove belt; try not to trap fingers.
13mm spanner undoes tensioner, 15mm spanner undoes idler (if you have aircon).
Refitting is reverse of removal.
You need offset spanners like these, ideally: Draper 8 Piece HI-TORQ Deep Offset Metric Ring Spanner Set Part No: 7
Refitting takes me between 30 seconds and about 10 minutes depending on how cooperative things are. If you have someone who can hold the belt in place to stop it jumping off while you're struggling with the tensioner, it helps.
You can also do it all from above by attaching a rope to the other end of the 15mm spanner and pulling on it. Smaky swears by it, but I've yet to master the technique!