Do I have to have a flak vest? Yes; a flak vest is what you will attach your chest armor to.
Does my flak vest have to have sleeves? No, but it is recommended as that is the best and easiest place to attach your shoulder armor.
Does my flak vest have to have a neck seal? No, you can have that as a separate item, or on the flight suit, or you can wear some other form of neck covering (such as a balaclava). Your neck should not be visible when your helmet is on.
How should my flak vest fit? Your flak vest should be fitted to you over your flight suit, not baggy, and the length typically ends about at your belly button. If your flak vest has as zipper, that will need to be covered with a flap or something so any zippers are not visible.
Does my flak vest have to close in the back? No, you can have it close any way you want. The back is the most common, followed by on the sides. In the front is not recommended as you will be able to see the closure under the armor plates.
What should I make it out of? As said above, this is what you’ll be attaching your armor to so it needs to fit WELL and be strong enough to hold it. A flak vest should be made out of any thicker, sturdy fabric like duck cloth, canvas, leather etc. If you have a shirt or vest that fits well and have some basic sewing knowledge you can even use that as a pattern.
There are many ways to get or make one. You can commission one/purchase one already made to your measurements, you can make your own. If you make your own, you can either use an existing pattern, a pattern you made, or you can modify an existing garment into one.
There are many great vendors around for flak vests on the forums, Etsy, etc.
A prolific vendor on the MMCC forums is K'raam Xrati. You can find his for sale thread here. They are $95/each and you have a good selection of colors.
You can also find their Facebook page here.
UPDATE: it has come to our attention that this vendor currently is months behind on orders and communication has been minimal, yet quality remains high. We recommend caution before proceeding with purchasing depending on your timeline.
There are patterns for flak vests specifically made by other members of the mando community, but you can also make your own pattern or modify an existing pattern of garment to make a flak vest.
For fabrics: The best recommendation is a sturdy fabric such as a duck cloth, which you can readily get at JoAnn fabrics (like this kind HERE) or other fabric stores.
There is a great tutorial and pattern for making your own on the Dented Helmet here.
HERE is a good tutorial on the Mercs forums by member Pointoforigin.
HERE is a good MMCC forum thread on modifying a found vest into a flak vest.
If you want to make your own pattern, you can start from scratch or modify an existing pattern. A good one to use is something like this one, though this is an older pattern and not as easy to get.
Why this type of pattern? It has a mandarin style collar on it, which works perfectly as a neck seal. The front seams (called princess seams) are great for adding shape, especially for women. It also has sleeves which you can shorten easily, and you can change the closure to be in the back instead of the front.