The phrase is:
あたしたちでつくっちゃえばいいんだよ、運命{うんめい}なんか。
So far from what I understand:
あたしたち is a feminine version of "we",
つく would be 作{つく}, which is "to do something"/"make something", I believe,
and ちゃう is "to do something completely".
However, from that point on, I'm confused. I don't understand:
If someone could help explain these two points to me, and/or the whole meaning of the sentence it would be much appreciated.
Yes, あたしたち for "we" and つく for "[作]{つく}".
ちゃう is the casual form of てしまう, whose meaning is to do completely.
ちゃう/てしまう involves feeling of "solve the problem by making it completely done" or "we don't have be concerned with it anymore after it is completely done."
In your sentence, it can be inferred that the speakers are trying to solving some problem by making [運命]{うんめい}.
Next, the phrase ばいいんだ, ば is for supposition ("if we did ...") and いい is for "good" or "OK." ん is the casual form of の, and のだ is for emphasis.
So its word-to-word meaning is like "It would be OK if we did ...". One of its better translation is: "What we have to do is only to ..."
Lastly, the reason 運命なんか is placed at the end is what we call [倒置法]{とうちほう}. As the comment above says, it is not hard to understand since we sometimes do it in English.
