Now that I’d ordered my kit, I could just sit back and wait for it to arrive, right? Wrong!
Most of the kits you buy include only the major plastic parts of the costume. For something like a stormtrooper, that’s the vast majority of what you need since you’ll be covered almost from head to toe in plastic. But all costumes need more than just the basic kit. There are blasters or lightsabers to add and then there are all the little detail parts (known as greeblies) that you have to find. Websites such as Etsy are littered with parts. More and more of the suppliers are including all the greeblies as part of their kit (or can send them as an optional extra) but, in most cases, they are not as accurate as the people who specialise in a specific part. 
One word of warning, there are many people out there who say their parts are approved by the 501st. This is not exactly true. Even if the parts are good enough, they still have to fit your costume, be installed correctly and so on. Worse, there are many who are just plain lying. Always ask around before ordering to make sure your supplier is reliable.
Nowadays, with the advent of 3D printing, it is becoming easier and easier to source all the detail parts. Some makers are even sharing their 3D files online so you can print your own.
Weapons are a whole other headache as it is not only the look of them that needs to be considered. What should they be made of? Resin models are often cheaper but tend to break if you drop them. Metal looks and feels amazing but weighs a lot more. If you are having a lightsaber, do you just want the hilt to hang on your belt or do you want a “working” blade? If you are having a blade, should it be detachable for belt use? Should it be strong enough that you can actually fight with it?
For the soft parts (ie anything made from cloth), you can choose to make it yourself if you have the skills (or a nice friend with the skills). That saves a lot of money and also lets you get started while you wait for everything else to come in. However, it is not an easy task. Often there are no patterns or it is hard to find exactly the right type of cloth. My suggestion is that before you buy any cloth, take photos and get it checked out by someone else. Many costumes have measurement / material guides such as the ones below.
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The newer costumes have often been displayed at exhibitions, Star Wars Celebration etc and have official lists of what materials have been used. You certainly can’t get any more accurate than that!
There are also people who sell soft parts and do a very professional job. You can expect to pay a hefty sum but you will usually get what you paid for. People like Clothears Costumes make lots of different parts and are even willing to discuss custom jobs.
As with everything else, the answers to all your questions can be found on your local garrison and costume detachment forums.
The best way to make sure you have everything is to read through the Costume Reference Library for your chosen costume. They are super-detailed and will point out anything you may have missed or forgotten. Once you are certain that you’ve ordered everything, you can finally sit back, relax and wait for it all to roll in.