Hello Rubia,
Your question:
"How do I say: How do I use...?" (in Japanese).
OK, here is a quick and easy answer for you.
どうやって使いますか
「どうやってつかいますか」ー>(dou yatte tsukaimasu ka?)
どう = How
やって = Do
使います = Use
か = marks it as a question
So if you wanted to say:
"How do I use this?"
Just say:
これをどうやって使いますか
Cool?
Got it?
Rock On!