Intensive pronouns and reflexive pronouns are very similar. The only difference between them is that the intensive pronoun emphasizes or intensifies the effect. Intensive pronouns aren't needed in the sentence to make it grammatically correct, it just adds a "flourish." In contrast, if you take a reflexive pronoun out of the sentence it won't make grammatical sense.
1. I baked a cake for the president himself!
In this case "himself" is the pronoun. To see if it is reflexive or intensive let's take "himself" out of the sentence and see if it still makes sense:
I baked a cake for the president!
It does make sense! In this case "himself" is not needed in the sentence, it just shows the wonder of baking a cake for the president (an undeniably important person). This makes this pronoun intensive!
2. He gave himself plenty of time to study for the test.
Let's remove "himself" from this sentence:
He gave plenty of time to study for the test.
I would say that it doesn't make sense with out "himself." Who did "he" give the time to? We don't know with out the pronoun "himself." This makes this pronoun reflexive!
3. I myself will move the couch.
Let's remove "myself" from this sentence:
I will move the couch.
It does make sense! In this case "myself" is being used as the emphasis that they will do all the work of moving the couch without any one else's help. This makes this pronoun intensive!
Hope this helped!
~Just a girl in love with Shawn Mendes