How do I use "preparar" and "preparar-se" correctly?
What are the most common ways of using these? What cases need only "preparar" and which need "preparar-se"? I'm also unsure about reflexive verbs in general even after reading many descriptions online, maybe I need somebody to explain it to me like I'm a six-year old. Thanks!

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Dear Andy
The verb preparar is often used as a way to express an action of a sequence e.g to cook or build something
-Preparar o prato ( to prepare/make the dish)

The verb Preparar-se is reflexive and shows that one is preparing him to something ( like a competition, an audition...)
Eu estou me preparando para a competição ( I'm preparing myself to the competition)

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It also can mean to cook.
Estou preparando o almoço
I am cooking the lunch.
Mãe, a senhora já preparou a janta?
Mom, have you prepared the dinner?

Nestes casos podemos usar também o verbo "fazer".
Eu já fiz a janta.
Eu vou fazer o almoço
Eu estou fazendo o almoço.

Só não fica muito legal dizer "vou fazer o café da manhã", melhor é dizer "vou preparar o café da manhã"

And another thing: fazer um lanche means to eat something light. Like biscuits, milk, bread, fruits...

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Hy Andy. Preparar-se means brace yourself; get ready. E.g:
Prepare-se para correr
Get ready to run(be ready when the time comes)
Vou me prepara para a festa
I'll get ready to the party(I'll put some good clothes
Estou me preparando(psicologicamente) para a conversa.
I am getting ready(psycologically) to the conversation.
Notice that I wrote "me preparando" instead of "preparando-me".  It's the way we talk imformally.

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Generally speaking, you need to use the reflexive form of the verb when:

1)The subject is doing an action to him/her/itself; 
Examples: "Eles se beijaram". They kissed (each other).  "Nós abraçamo-nos" We huged each other. "Preparem-se." Preapare yourselves.

2)Some verbs do not excatly follow the rule number one, but you just need to remember that they are reflexive verbs.
Examples: sentir, parecer, 
"Eu me sinto tão triste." I feel so sad.
"Eles se parecem com o meu pai." They look like my father,

Remember: "se" it's not the only particle that makes a verb reflexive, there's a table with which one should you use. Eu=me, Tú=te, ele/ela-/você=se, nós=nos, vós=vos, eles/elas/vocês=se

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I don't know if  you understand what I mean. So I will write some setenses to you see:
Ele foi ao banheiro preparar-se para o discurso. 
Ele está banhando para se preparar para dormir.
O Brasil durou sete anos para se preparar para a copa do mundo.

Minha bisavó vai preparar o maior banquete que essa família ja viu.
Dura duas horas para preparar a lasanha.
Eles duram oito meses para preparar um episódio dos Simpsons.