The biggest thing to focus on is a narrow audience. Pick only a couple keywords that you believe can drive the most revenue and focus all your budget there.
Another part of this is looking at long tail keywords. The keyword “shoes” is so broad, some of the people that click on your ad because the keyword “shoes” triggered it, might not be your idea customer.
But, if you use the keyword “red women’s shoes open toe”, you can that customer is looking for what you have.
In addition, long tail keywords are generally much cheaper than broad keywords. The traffic is less, but that’s okay if it’s your perfect audience.