just scroll to the bottom of the page, you should find a drop menu that says '1 out of 5' when you click on it it'll drop down, choose what score you feel it deserves from 1-5, then click vote. if it's a non-working spell, not really a spell, or dangerous [trust me, there have been some] click the 'mark this spell as spam' found just above the drop menu on the right. also found just above the drop menus is 'favourite' and 'print' in case you wish to do one of those options.
if you notice a spell that is dangerous, [i.e. stab yourself, jump from really high, ingest poison] has profanity, or breaks any major site rules, inform the mods or Pet immediately and it will be taken down almost instantly. [seriously, it's shocking how fast those spells disappear]
this system is good and bad. good because everyone can and should rate high good working spells, and flag/rate down fluffy spells. however, since anyone can rate spells, the reason some fluffy spells are high is because someone posts the spell, then using their other accounts, or asking friends [or both] rate it 5 out of 5 so much, it's hard to rank down. point is, while this system can help you 75% of the time find good spells, you still need to use your head.