Como dizer "ja sei/sei como é..." em inglês

Hello, Rined.
O melhor seria se você colocasse frases completas, mas aí vão minhas sugestões.
Já sei = I already know it./ I already know that. or Good idea!(Como se estivesse pensando numa coisa nova: Boa ideia, já sei o que fazer.
Sei como é. = I know how it is like./ I know how to do it.
Já fiz aquilo = I've already done that./ I've already done that before.

Adding...

Sei como é... = I know how it feels like. (num sentido de compreensão)

extra..

Já passei por isso, já aconteceu comigo = I had that happen to me>> (once, when i was a child, in the bus, in the elevator and so forth)

see ya

Pessoal,

Sugestão:

I know what it's like.

Well,I know what it's like - I've done all that and more.[www.seamedia.com.au]

Bons estudos!

Boa sorte"

And...

A common colloquial way to express "Já fiz aqui, já passei por isso" is Been there, done that.

I am broke and I can't find a job.
Been there, done that. Many times I have been broke and without work.

My girlfriend left me and I feel so loney and sad.
Been there, done that. It's awful to get dumped when you're in love. It's happened to me too.

I went to Argentina last year and worked in a bar for five months.
Been there, done that. I spent four months in Mendoza working in a restaurant back in 2005.