It's much simpler than you think. When someone or something does something does something to theirself or itself, you should use a reflexive pronoun. In other words, when a verb's object or indirect object is the same as the subject, then you should use a reflexive pronoun in place of the object. Ex. "He killed him" - This means that there were two people, and one killed the other. "He killed himself." - This means that there was one person, and he comitted suicide. It's exactly the same with prepositions. Ex. "I looked at myself." - ie. the speaker was looking in a mirror or was looking down at theirself. "To do something by oneself" means "to do something alone or without help". From what I've told you, you should be able to figure out why. Some languages use a reflexive pronoun with a verb to make a phrasal verb (ex. "I feel myself" might mean "I feel" in that language). Fortunately that isn't done at all in English, so you don't need to worry! Sometimes a reflexive pronoun is used at the end of a sentence to emphasise who did something. Ex. "I saw it myself!" "The king did it himself!! You could remove 'myself' and 'himself' and the sentences would be exactly the same, but without the emphasis.