[ˈjanə] I was wondering if it's spelled ヤナ, or maybe because the stress is on the first syllable there should be a dash board: ヤーナ?
Depending on how much Japanese you already know, my answer might come as a surprise. I am going to say that either 「ヤナ」 or 「ヤーナ」 would be fine so long as you yourself are comfortable with it.
Unlike the far more commonly seen or heard katakana names such as 「スミス」、「ジョン」、「イザベル」, etc. that are already widely recognized by the general public as "correct", very few people (if anyone at all) would feel strange upon seeing or hearing either 「ヤナ」 or 「ヤーナ」. If the "ya" part of "Yana" is elongated enough in your native language, then you might take the latter.
Katakanizing foreign proper nouns is not rocket science; It is basically phonetic approximation. The example I always use when talking about this is the name "Charles". As an English name, we write it as 「チャールズ」 and as a French name, 「シャルル」. The original alphabet spelling has little to nothing to do with the katakanization. It is the pronunciation that actually counts.