| Jewelry and Watches

The follow up to the most important question a man can ask a woman, "Will you marry me?" should ultimately be: "Did I do a good job?"
That's what the man is thinking when the woman says "yes" (we hope, right guys) and she's staring at the ring with love and affection.
Yes, she loves you, too, but she's admiring her newest love: that engagement ring.
But for most guys, thoughts turn to self doubt in some cases, wondering if she likes it, even though you ask her a million times over and she keeps shaking her head affirmatively.
Before you reach that point, you should be planning and thinking and then thinking and planning again about how to secure the perfect ring, the one that will be stunning and exactly what she's wanted from you since that first date or the moment she started thinking about life with you forever.

So how exactly do you buy the perfect engagement ring? 
The word "perfect" isn't the same for everyone. Your girl might be dropping hints at a princess cut and wedding band set, but you've decided to go against that and get her the one that is the higher carat.
Typically, guys should stay extremely close to those hints and pay close attention to what they're telling you about the ring that is the one of their dreams. Women will ultimately tell you what they want, but make sure you steer clear of actually taking her ring shopping. That's tacky if she picks out the ring she wants.
In some cases, that works because everyone is happy but it loses the spontaneous nature when she's already expecting a ring that she saw once before, no matter how much effort you put into the execution part of it.
It also doesn't hurt to take all the information you've listed intently to and take it to a professional or at least confer with someone who has done it before. Being a novice at it isn't anything to be embarrassed about; this is a moment you'll remember forever, and you can't take it lightly and just assume you'll "figure it out."
Finally, don't be cheap. No, you don't have to spend your entire life savings, but you also have to remember that you should spend what you can afford. That means you shouldn't take short cuts with clarity or cut and if finding what she wants means spending at your just above you budget, then make it happen.
The end result should always be your wanting to please your soon to be fiancee and show her that this proposal is hardly formality mixed with little to no attention to detail.

Related:
Rough Cut: Diamonds Are A Girl's Best Friend But Buying The Wrong Engagement Ring Could Be A Nightmare For The Would-be Groom
Seiko & Destroy: Finding The Perfect Timepiece Means Putting The Time In To Find The Right Mix Of Price And Practicality And One Brand Does It Better Than Most
Gold-diggers: Golden Opportunity Meets Questions About Which "cash For Gold" Store Actually Delivers On Promises Of Treasure
Rented Bliss -- Wedding Jewelry Often Is Best Worn When It Isn't Yours