Can anyone offer me a step by step for changing all of my belts? Is there an order to follow? Do the pulleys act as tensioners like in todays cars?
Thank you in advance!

a step by step is found in the Shop Manual. The pulleys do not act like tensioners in the Series 3 XJ-6, but they do in 1960's Jaguars, sprung pulleys are not a modern invention, they were invented in the late 1950's.

the order of belt removal is front to back, so if you need to replace the belt closest to the engine, you'll need to remove the other belts in front of it first.