I have never had a mpg gain from a valve adjustment. I suppose it is possible that you could gain mpg if your valves are way out.
To adjust the valves, you need an engine barring tool (from Snap-on, Cummins, etc), the ability to find TDC, . 010 & . 020 feeler gauges, some metric wrench size I can't remember and a flat blade screwdriver. You may need new valve cover gaskets depending on their condition and how easily they come off.
Some say you can turn the engine over using the alternator nut. You can but it turns the engine in the opposite direction of normal rotation. You be the judge if that is the method you want to use.
Good Luck