The basic unit of play in SeeD is the Campaign - a group of players, guided by a gamemaster, playing through an adventure. Campaign creation consists of the answering of three questions - "What is the story about", "Who are the main characters", and "How shall we play it".
Story
Deciding what the story is about means deciding what the setting is and what the setting is about, and where the party is and what they are doing at the very beginning of the game. It might mean deciding some flow of the plot-line in advance, or it may simply involve creating a world full of conflict and adventure for the players to explore.
It also means determining the tone of the game - whether the world is grim and cruel or bright and heroic, whether the supernatural can be approached rationally or if it is best treated as shrouded and mysterious, whether reaching 0 HP means nigh-certain death, grievous injury, or simply stumbling to a knee and giving up. A tone should be chosen that makes for a compelling tale, but also one that the players involved are comfortable with. It might be useful to discuss tone in terms of other games that the group has played - "Like FFTA2" is a useful marker, as is "Like FF6".
The setting should be detailed enough to allow the players to get a sense of what the game will be about and what sort of party they should create for it. A few paragraphs about its recent history, a list of some of its immediately important nations and cities, an explanation of the role of the fantastic, and a general overview of the world's culture and intrigue should suffice, though more can be added to taste - detail can always be crafted as the game continues.
Party
The players should decide who the main characters are by creating a party together, guided by the rules governing character creation in Chapter 3. Each player generally controls a single character, though one could create a campaign where the players switch from following and playing as one group of heroes to another. Given the fast pace of combat in SeeD, it is probably not a good idea to have players controlling more than one character at a time - it could become difficult to keep up.
Players should aim to create characters that they'll enjoy controlling - they should be able to go about the adventure in a way the player wishes to, they should mesh well with the atmosphere and setting of the game, and mechanically, they should be fun to play in combats and during their travels.
Mechanics
As well, the mechanics of gameplay should be decided upon, the question of "what systems will the campaign use". Generally, a game in SeeD has one system for character growth, one system for equipment, and one system for skills. These serve specific purposes:
Growth: Awards characters new abilities as they gain levels, diversifying their tactical options and granting them power.
Outfitting: Increases the physical damage and defenses of characters as they progress through the game, and grants them special properties.
Skills: Models the expertise and prowess of characters outside of battle, allowing players to define how their characters adventure.
A campaign should have one and only one system of each type. In this ruleset, the "assumed" systems are the Job, Equipment, and Talent systems, and they rely on one another. Other systems specifically state how they interact with the assumptions of these systems.
The group is welcome to add other systems too - Some will be included in this ruleset, others might one day be found in the community, and an intrepid gamemaster may wish to write one specifically for a campaign!
Systems generally make characters more powerful, and the more additional ones are used, the easier time they'll have in combat. This is why it's important to apply systems to the entire party - two systems of the same sort aren't guaranteed to provide an equivalent amount of power. If many are added, the gamemaster may wish to explore the very limits of monster creation, seeing how difficult they can make their antagonists can still be defeated - or you might enjoy a more relaxing campaign that might be easier than another one.
The group might wish to think about how the mechanical systems used relate to the setting, if they relate at all. In some video-games, the plot directly addresses the fantastic powers the characters are wielding, and how they wield them. In others, job change is just something taken for granted. Depending on the setting, the mechanical quirks of the campaign can be swept under the rug and ignored, or brought front and center, explaining the characters as people with unusual power gained in a specific manner.
Scheduling and Format
It's also important to discuss how the group will meet to play - Be it over an internet chat service, on a table, how often they will play, how long the game will continue, the time of each session, and so on. This game, like all others, generally works better if it's played at a time where everyone is comfortable, focused, and not rushed.
SeeD can be played as an epic adventure ranging from levels 1 to 99, but it's also designed such that it's mechanically interesting across all levels, as abilities are gained fairly quickly in most growth systems and are disassociated from level. As such, the game may start and end at any level the group desires, and it might be fun and rewarding to experiment with a variety of different paces.
Try a heroic, world-saving adventure in eight sessions, for example!