it gives the illusion to others your nose is smaller, or your lips are bigger, or whatever you cast the spell over. it doesn't change anything, you probably won't notice a difference, and those you're close with probably won't either, but random people in school/work/on the street will. of course like all spells it'll wear off in time. i've never done any illusion spell, so i don't know how effective they are.
glamours, like said, use illusion to cover up things that you don't want people to see. in the basic mechanics of it, it's just like fashion. you want to enhance your better parts to draw attention away from the bad parts. and another main thing is to make sure you actually feel beautiful, insecurities weaken glamours and often causes them to fail
for example: i would focus on my face, and using the illusion of perception to make it more attractive, to balance out the size of my stomach, or make up for it. but you have to FEEL as though you're face is more beautiful. confidence is the key