1. DON'T LEAVE IT TO THE LAST MINUTE
It is important to take at least a month and a half (to be safe) to start looking. It takes a bit of time to look for the perfect diamond and probably you would want to compare with few other places before putting down your deposit. Crafting the ring on the other hand takes time as well, if it's a simple solitaire, it could be done within a week or a week and a half, but if you would like to have a special design, then it will take at least 3 weeks to craft, depending on the complication of the design.
You should ask yourself what budget are you comfortable working with? This will give a good idea for the jewelers to work within your budget. If you're not sure, then think of how much money you could spend without worrying!
3. DECIDE ON THE CUT
Do some research on what cut you want to go for, when in doubt, bring your other half's best friend or sister, they will probably know best, girls talk to each other about anything...!
If you would like to do it on your own and you have no clue, go with a round brilliant cut diamond. Round is a great choice and it's classic, can't go wrong with round! It will be a hassle if the jeweler needs to find more than 2 cuts for you, narrow down to 1 or 2 cuts because looking for diamonds requires a lot of time and research.
4. BUY FROM A TRUSTED JEWELER
Only buy your diamond from a reliable company/jeweler. It's best if you find a jeweler that your friends or family recommend. Do not be scammed! There are rumors where some of them switch stones or sell counterfeit diamonds or selling you a stone that has a GIA certificate and a replicated GIA inscription on the diamond that matches the certificate, but the actual stone does not actually match the description (in this case, only experts could see the difference).
5. COMPARE DIAMONDS
Do compare few diamonds next to each other with the different 4Cs: carat, color, clarity, and cut. It will be good to see the differences in carat, even if it's only 0.10ct difference between the diamonds. You could see a significant difference in color if the diamonds are at least 2 grades apart. For clarity, you'll be surprised you will see a diamond nicer than the other diamond within the same grade (it will be more obvious if it's in the SI category).
If you're looking at a round diamond, only look for Excellent cut and none other, excellent cut will provide the best light of return and sparkles the most. If you're looking at fancy cut diamonds, then you will need to see which one you prefer, for example, if you're looking at oval cut diamonds, you could decide if you want a long a slim oval or a short and fat oval. This is why fancy cut diamonds don't come with a cut grade in the certificate like the round.
All these variables will affect the price a lot!
6. COMPARE WITH DIFFERENT JEWELERS
Shop around and compare with 2-3 other shops or jewelers. You will probably see nicer diamonds in other places, every piece of diamond is different even when they have similar gradings.
DO NOT go for the cheapest one because it is cheaper. If all the diamonds are presented to you within your budget, you should go with the one which you have a connection with most. I always say, GO WITH YOUR GUT FEELING!
Besides buying a beautiful diamond, craftsmanship is very important as well to make a beautiful piece. Whether it is a simple solitaire setting or a fancy setting, it will totally give a different feel even if it is used with the same diamond. You cannot be last minute and rush this part. Craftsmanship is everything to deliver a beautiful piece. If they said they could finish within a day, even if it's set on a cast ring, do stay away from them. Even setting a stone into the ring takes a lot of time and patience to make all claws equal and held down on the stone securely. I tried this stage myself with the Van Cleef & Arpels half-day course in Hong Kong, I understand now why you cannot rush craftmanship.