に慣れる【になれる】

に慣れる means "to get used to": something that was new or hard stops being so. The thing you get used to takes に and goes straight before it as a noun. A verb takes の first.

The thing takes に, never を, and a verb cannot take it bare.

• × 日本の生活を慣れた → 日本の生活に慣れた
• × 早起きするに慣れた → 早起きするのに慣れた

慣れた or 慣れている

慣れる is a change: the moment you become used to something. "I'm used to it" is the state after that change, and takes なれている慣れている. English "am used to" is present tense, but 慣れる on its own would mean "will get used to".

まだ慣れていない

"Not used to it yet" is まだなれていない慣れていない. The past negative would say the change never happened at some point in the past, which is not what you mean.

• × まだ慣れなかった → まだ慣れていない

慣れる and 慣らす

慣れる is intransitive. Its transitive partner ならす慣らす means to get something else used to a thing, often your own body.