I always put a ring spanner on the sprung loaded tensioner and the other end of spanner attach a ratchet strap. Locate other end of strap to a secure point and then ratchet until tensioner is pulled clear of belt. When new belt fitted release tension of strap so tensioner is back on belt. Job done.
........and if you haven't done one before, you had better draw yourself a picture which will save you time staring at it when you can't remember how it went....back in the old days, there was only the water pump and dynamo to go around but some belts now have a dozen different pulleys to go around and can be confusing?.....just saying.