vents can be either purchased or made yourself. they can be fastened to or built into the enclosure. they can be round or square. vents can be contained inside the box, or they may be routed outside.
vents consist of two parts - a port and a duct. the port is the actual opening, or mouth of the vent. it is often flared (horn-shaped) to reduce air noise. the duct is the tube or tunnel that extends a certain length into or out of the box. the size & length of the vent determines what's called Fb, or the tuning frequency of the system.
JL audio used to (and should still) make a venting system called "flexport" that consisted of a flared port and flexible tubing of variable diameter. you just glue the port into a 2.5-4" hole with silicone or liquid nails and screw it down. the tubing just slips over the back of the port (you can glue it together too, but i never did).
i also remember something called a "lightning port" sold in crutchfield a will back. i think lightning audio made it. the lightning port was an adjustable duct with a flared port on each end, built all as one piece. you could simply mount the vent and adjust to the tuning frequency by varying the length. i haven't seen this item for a while, but they may still carry them if you ask.
as for the D.I.Y. route, you can make ducts out of PVC plumbing pipe, but the hard part is creating a port opening that won't make air noise. using big vents or multiple vents can help here, and you might try searching on ebay for more aerodynamic ports - i'm sure a few companies still make them.
square vents can be easier to make, assuming you are building the entire enclosure and can plan everything all at once, and if you make the area of the vent large enough, it won't make noise so you won't need to worry about making a port.
usually, square vents are built as part of the box, almost like an extra chamber except it is open to the outside. since this type of vent is not removeable, you may need to make the box so that it can be disassembled for tuning purposes.
also, say if i used a manufactured port or a PVC pipe, i would glue and screws the tube in the wood or w/e material. but with the bass us high wouldnt that like move or wobble the duct inside the enclosure to a point where it might fall out? Or is the duct short another were it the bass wont affect it. Does the size of the box determind the lenght of the ducts. Like say if i were to have 2 15" sub and 2 10" subs would the ducts have different lenghts?
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also, say if i used a manufactured port or a PVC pipe, i would glue and screw the tube in the wood or w/e material. but with the bass us high wouldnt that like move or wobble the duct inside the enclosure to a point where it might fall out? Or is the duct short enough were it the bass wont affect it. Does the size of the box determind the lenght of the ducts. Like say if i were to have 2 15" sub and 2 10" subs would the ducts have different lenghts?
if you don't practice good craftsmanship, yes, your vent could fall out. if you build everything nice and tight-fitting, you'll be fine.
you can also use plastic cement to bond pieces of the vent together so they will not come apart. for long ducts, you can make an internal brace out of wood to support the end.
vent size is determined by the sub, the box volume, and your objectives. if any of these things change, the vent should also change or you will notice your box will be out of tune.
without going through the entire process of vented box design, you can safely assume a few things:
1) bigger subs move more air, which means using larger diameter vents will reduce port noise.
2) making the duct longer will lower the tuning frequency. shortening the duct will raise the frequency.
3) making the diameter of the port smaller will lower the tuning frequency. making the diameter larger will raise the frequency.
4) increasing box volume will lower the tuning frequency. decreasing the volume will raise it.
5) the tuning frequency can be determined by what woofer you are using and what kind of sound you want from the system.
if your goal is to learn and have fun, you can experiment with different box/vent/sub combinations until you find what you are looking for. if you want to build a good box the first time, i'd go to your local library and check out a few books on vented speaker design.