-TRP3141592
Use a big socket wrench/breaker bar and the correct socket/bit. Some tensioner pulleys use sockets, others torx bits, and some just use a 3/8 square hole.
The other answer is correct as well, but on my 3.1 litre the tensioner pulley is attached to an arm and the bolt comes through from the back side which makes it impossible to reach with a socket. What I use on mine is a crescent wrench and adjust it to fit the width of the arm the pulley is attached to. This arm is square and the wrench slides on perfectly and then I just pull the wrench up. You can also use an appropriately sized wrench but I don't know exactly what size it is.
Tom
On my 3.8, I used a breaker bar and socket to release the tension and then fit the belt on.