Sometimes it's a bit difficult to decide on which of these two words to choose. Let's start by saying that like is a preposition: it can then be followed by a noun, a pronoun or -ing ending.

What a beautiful lady! She is like a fairy...

You can also find the word like in the structure  'like (someone /something) -ing.'

That sounds like a wife shouting!

Now will take a look at the word 'as': we use mainly it before a subject + a verb.

Do it exactly as I told you.

Do it like this

Do it as I told you...

The word as can also be a preposition, so you can use it with a noun. But the meaning is not the same as like.

we use as + noun  when we want to talk about someone's job or how we use something.

When I was an adolescent I worked as a waiter.

We use this place as a laboratory.

I work as a slave (=soy esclavo)

I work like a slave(=trabajo igual que un esclavo)

As your teacher I advise you to study (=soy tu profesora)

Like your teacher, I advise you to study (=no soy tu profesora, pero te recomiendo lo mismo que ella)