Hm, personally I do not like the idea of tattoo in a language you do not understand. You might get what you do not really intend to have and you cannot tell because you do not know the language yourself... I've seen a lot of awful kanji tattoo on internet (a lot of people make themselves a laughing stock without knowing) and the problem is, unlike T-shirt you cannot change or delete the tattoo once you've got it, whether you like or not.
But after all it's your skin so if you really want it, I just put some information for that:
1. Japanese usually read from left to write when written horizontally. (Same as English and other European languages.)
2. 永遠 (eien) means eternity.
3. 自由 (jiyuu) means freedom, liberty.
4. Putting these two words together does not make a sentence or phrase.
People can understand that you have two words on your arm, "eternity" and "liberty" but it does not have the same effect/connotation as the English phrase "freedom forever".
5. If you still want to have it, 永遠に自由 meaning "(I'm) free forever" might be one of the options. Japanese langauges does need particles and okurigana which are not kanji but kana, it cannot be written with kanji only. (If you want a phrase/sentence all in kanji, I'd recommend to go for Chinese not Japanese.)
But hiragana (に in this case) among Kanji do not really look cool.