The timing marks on the cam gears line up with the top of the head where the valve cover sits. The gears are coarse enough that if you're off one tooth, it'll be obvious you're off.
Take some time rolling over the engine before taking the cam out (if you need to take the cam out) in order to familiarize yourself with the marks.
Also, pay attention to the marks on the crankshaft that are revealed after removing the circular cover at the crank end on the cam side of the engine.
Cam chain tension is maintained by the spring tensioner you manipulate with the "key". Once things are back in place, un-wind the "key" and spring pressure takes care of the rest.
I plan to die young, as late as possible.