4ltr. Thor serpentine belt tensioner

Hi all, I'm new to this engine and am trying to replace the tensioner and alternator bracket.
The problem I have is I've no idea how to slacken the tensioner off to put new belt on. I've read that you need a 13mm spanner and it's easy but I'm feeling a little slow, not to mention cold, atm and just can't see what to do with the spanner!
Thanks in advance for any help.

It's easier to take the fan shroud off first. Locate spanner onto bolt in the centre of the tensioner & turn clockwise. The tensioner should move with it to slacken belt. There is alot more room to do this with the fan off but that can be another headache.