This is the most expensive part of the engagement ring, so it makes sense that you can easily calculate your budget with it in mind.
Start with the Diamond
This way, you are effectively limiting how many diamond settings there are to choose from. In this order, next comes choosing the type of metal and then the ring style. Engagement rings can be set in yellow gold, rose gold, white gold, or platinum. Each of these metals brings its own benefits, but you should also consider how it would look on her hand.
Knowing your Partner
This is a key part of designing an engagement ring yourself. For instance, if she has tastes align with the traditional styles, maybe you should go in for a 1 4 carat solitaire diamond ring. For vintage styles, consider choosing something set in rose gold.