How do I know when and where to put an accent on a word?

Accents allow you to know what syllable needs to be stressed in a word. There are a few general rules as to what syllable is stressed in a word such as if a word ends in a vowel or a vowel + s/n the stress is on the next to last syllable. But there are exeptions to this rule and in those cases, the vowel on the stressed syllable has an accent. For example if you write the word "caminando", you know this word ends in a vowel and you know you should stress the next to last syllable. Therefore you know you don't need to put an accent on any vowel of this word. On the other hand, "camión" is a word ending in a vowel + n and you know the stress does not go on the next to last syllable and therefore the word carries an accent on the vowel of the stressed syllable. In this case, the O.
I would then elaborate on the entirety of this answer but there are many other rules and explanations that would require a long answer while making sure the student understands every part. This is a long and complex subject within spanish that would require at least an hour to properly teach and explain and multiple lessons to learn and master how to properly apply the rules.