This question may scares some of you a little bit, but do you really know her ? Don't worry we're only talking here about what she likes and dislikes, and some more useful little things but that are very important.
Very important like her ring size. Do you know it ? It is of course something to know before you choose a ring, even if you can resize it later.
Each store we recommend gives you a ring sizer you can download and use to measure the ring size on her finger in more than one way. Well if you had to measure her finger it should be subtly (while she sleeps ?). You can also use a ring from her jewelry collection.
Another thing to know is her favorite metal color. Some women only wears Yellow Gold jewelry, so her engagement ring should be of Yellow Gold. Why not ask her ? Subtly of course, in front of a jewelry store, watching what's on display.
Another tip : her family or best friends may help you to answer this kind of questions. If they don't already know, they can inquire about it (SUBTLY again).
Next step, choose a ring.
The steps to create your own engagement ring :
Establish your budget