Open Civ5, start a map, then start fire tuner. If you have two monitors, you can simply place the fire tuner window in the free window and gain access to its tabs for plopping units, resources, improvements, experience, etc. You can also tell it to 'autoturn' (i.e. play all the civs for you) for a set # of turns, thus freeing you up to test your mod without 'testing' (i.e. playing) your mod for hours on end.
If you don't have a second monitor, you can either put the game in windowed mode or simply alt-tab back and forth.
Me? I alt-tab its not so bad.
The Luaconsole on the first tab lets you mess with the front end of the game's data (diplomacy, units, GNP, etc.) and is the most useful part on an informational level. There are multiple mods that take advantage of the Luaconsole- see this one:
http://forums.civfanatics.com/showthread.php?t=392206&highlight=tuner
Its pretty simple- just play around with it for awhile.
The key is to have the game running - it won't do anything unless the .exe is booted and in a game.
G