I am not going to tell you how the game works, rather I will inform you of what I currently think to be the best strategy. If you know nothing about dominion the following video will give you decent enough of a background:
There are three things particularly special about this build. First of all, in order to win the perfect game of dominion you cannot expect to capture all five points and hold them. That is impossible. To win the perfect game you need to capture three points and hold them the entire game. This is doable. Along with this the second point is do not expect to hold points near their spawn. You will be overrun. Key word is hold, capturing is great but priority is to drop back and protect your three points. Third, this guide breaks down the five players into five different roles. If everyone fulfills their individual role, you will win.

The five different roles are the Roamer, the Defender of the South (DOS), an AP fighter, and AD fighter and a Defender of the North (DON). Also of note, it is very nice if either your AP fighter or AD fighter also fulfill the role of Disrupter. These roles will be explaned in much more detail in the next 5 sections.

Mission:
Undermine the enemies front line. That is, push any point that is not closely defended by the enemy team.

Solid Roaming Champions:
Evelynn (my favorite), Twitch, Shaco, Rammus, etc. [Either someone with stealth or really fast]

What not to do:
In almost every situation you should not confront the enemy. You take a position only if it is undefended.
Do not defend your own points (if they are pushing your points, their points will be open)

Build:
You want movement speed, and you want defense. In the case that you do get caught with your pants down capturing one of their points, you don't want to die, best you survive long enough to get away to attack a different point.

Also, very important, you want Priscilla's Blessing since at this point it is the only item I know of that speeds up your capturing speed. Once you appear and start capturing their point they will be crashing down on you hard, sometimes its sufficient just to neutralize the point and not necessarily capture it. If they have pushed and therefore you do have time to capture their point if only 1 of them comes to reobtain it I find it pretty easy to disrupt them without necessarily engaging with them. Building tanky will allow you to fend off one of them while under a turret owned by you. You can force more of them to come help in order to capture. This relieves great pressure from top and bottom allow them to succeed. #winning

Other Notes:
This is a rather easy and fun roles to fulfill. In most cases you will piss off the enemy team and you prevent their team from being able to push hard at your team.

You may not feel your effect directly but your teamates will notice the difference. You force at least one of the people from their points to pull back to that point. That relieves pressure from your DOS or takes one out of the fight from top giving your team a better chance of winning and holding top.

Mission:
Do not let the enemy team take your bottom point.

Alternative Mission:
ONLY if left completely open, take their bottom point but immediately return back to your own point to defend.

Solid DOS Champions:
Heimerdinger, TANKY Soraka, Mordekaiser, Singed, etc.
[Someone who can defend in two ways: not dying and having skills that do damage to disrupt capture (Often from a distance) ]



What not to do:
NEVER go to mid or top
never take relic buff
Push hard while alone (assistance will be discussed in the fighter chapter)

Build:
You want tankyness, at least in terms of health. Damage output can sometimes compensate for armor or magic resistance (like in the case of heimerdinger). With heimer you can place two turrets and sit back, splash damage from turrets will prevent capture and do quality damage. Use rockets and grenade to subsidize damage.


Other Notes:
If you are playing careful and still having trouble holding your point that is okay. This is when your fighters top as well as your roamer should be pushing and holding top and potentially their mid. Again, this game is not about holding all the points. The game is about capturing and holding three points and holding them.

Mission:
Kill your enemies, push and take top.

Alternative Mission:
Gank Bottom and Assist with Capture of their Bottom.

Important note:
This is definitely the most hectic and challenging role and unfortunately horribly important.

Much less important note:
These two should be only roles getting relic buff.

AD Fighter Champions:
Gangplank, Pantheon, Ezreal, Riven, etc.
[Your job is to kill. In most situations you will be 1v1, 1v2, or 2v2. You need to choose a champion that can excel in these kinds of fights. Burst damage and survivability]

AP Fighter Champions:
Annie, Brand, Karthus, Anivia, etc.
[Your job is to kill. In most situations you will be 1v1, 1v2, or 2v2. You need to choose a champion that can excel in these kinds of fights. Burst damage and survivability]

Its very beneficial for either your AD fighter or AP fighter to be a disrupter. This will be explained in the next chapter.

What not to do:
Do not leave top for too long.
Not communicate with your team
Go on suicide missions

Ganking and Support:
If your bot is having trouble holding your bot. This means you have a choice. Push hard to their mid in order to maintain three (assuming you have top). Or send your fighters bot to gank and if successful then potentially push and quickly capture their point.

NOTE: This is not a separate role but rather a category of champion that one of your fighters should fulfill. Fighting top will be a lot harder if they have captured it while you are running up there. So disrupt them from across the map so that when you get there they dont have it captured.

Mission:
If you are being backdoored by an enemy roamer. Or if one of your players is out of position and one of your points is being captured, it is beneficial for one of your fighters to have a global attack that will disrupt their capture.

Disrupter Examples:
Karthus, Gangplank, Ezreal etc.

A slightly separate catergory but also a valid option is if you have Teemo or Shaco on your team. Placing traps on the perimeter of your point can prevent the enemy from capturing your point quickly while you're away.

Mission:
Hold off pushes by enemy team until your fighters can assist you in dispatching them.

Alternative but IMPORTANT mission:
Prevent their roamer from stealing your mid point.

Strategy:
Oftentimes when top and mid are owned you can pace between the two keeping a sharp eye on the map and enemy locations.

Important Note:
If you need to go b to heal or buy do not leave top without notifying fighters so that they may cover for you.

Champions that could be the DON:
Kog'Maw, Lux, Shaco, etc.
[You want to be able to attack from a distance and have decent survivability]

Lets me explain:
Kog'Maw is fantastic, your job is not to kill them. Not without fighters, no. Your job is to keep them from capturing. With the huge range on his ult, he can continually hit them to disrupt capture. Lux is similar with the long range of her Lucent Singularity, she also has a shield and can immobolize enemies.

Build:
This position is a semi-tanky pseudo-fighter. You can build some tankiness but damage is just as nice. This character should not be sitting on a point and should avoid initiating fights without your fighters to help. Therefore they will be squishier than the DOS. Building damage will help fight off most pushes from the enemy.

What not to do:
Again, do not leave top without someone to cover for you. (Only for buying)
Do not think to go gank bottom because your fighters wont be as well prepared to defend top as you should be.
Do not get caught on tax fraud. Keep the books safe!

This is my first complete build but I am tired of queueing with so many people running around like chickens with their heads cut off. If you have feedback please give it. Also, I only put a few examples of champions that fit these roles, if you have ideas for how other champions can fit the roles please comment! This guide is more focused on the strategy than necessarily the builds I put in. Don't thumbs down this simply because of those because I definitely am willing to discuss changing some of those.