Using a string trimmer is pretty straight forward, however there are some extra little tips that may help you.  If your trimmer has a guard I would recommend removing it to allow more string out.  I usually leave about 8-10" on each side out.  Make sure the weedeater engine can sufficiently power the amount if line you have out.  Leaving more line out makes for a more even cut leveling out the imperfections of the trimmer moving around.  I also recommend using the largest diameter trimmer line possible.  Note:  removing the guard on your trimmer will void your warranty and is also dangerous...this article is for educational purpopses only and should not be attempted without proper safety equippment.