After making an account, you'll either be directed to start the season yourself or use the “Quick Create” option. Go through the motions, if nothing more than to be certain some key players are there. They won't matter much, but
It's better to be safe than sorry.
Picking a club comes with choosing 3 key players. These are more important cards that will be better than the average teammate. Pick a team and move to the next segment. It will show a lineup made from randomly selected cards. It's possible to reroll the cards given out, so try to land on something that has the best amount of level 2 players and a has decent pick for each person on a starting formation.
You should be given a complimentary set of star players when starting off, with one superstar. These are advanced teammates that may show potential, as indicated on their card. The superstar may be crucial to remember, but it can still end up being dismissible, if not compatible with the following steps.
Each club gets a giant amount of startup funds. There's no need to be conservative with initial spending, as the game doesn't involve a lot of money management. Instead, invest the large majority of the 300,000 or so credits to head to the Shop tab in the top bar and purchase a series of Basic Cards (2,000) and Premium Cards (9,000).
Try buying Premium Cards until you get at least one strong forward, a star midfielder and a solid defense position; anything else is gravy. Do also fill up on Basic Cards for more squad members and later use in training. They really aren't that less competent than the expensive option. It should be possible to fill out around 50 cards.
Depending on your winnings, you should now work towards settling on one formation, regardless of what the team was working on before. There are two ways to go about it. The most basic form is to choose a side where most of the star players with potential are active, as they'll be the biggest contributors. You'll most likely need to settle on some lower levels on other positions in the first league, given the lineup total is limited. Whatever you do, don't sell surplus members; you'll need them all.
If you choose to simply start potential cards, you'll still want to optimize the formation with similar players that share either a league, nationality or a team, indicated on their card icon. Shuffle these together, as their common traits create a bond between them.
A better option is to fill out an entire team of the same league by default or at least have as many eligible ones as possible. This also raises unity bonuses found in the second tab of the Squad lineup. If all cards are from the same league and eligible, usually through higher levels, all their attributes will be raised, making it a much more powerful squad than even just favoring a few stars. Any open gaps can still be filled with star cards from other leagues for added emphasis, but always make sure there is some sort of unity, as their match performance will significantly rise because of it.
Now, in the Role tab of the lineup, click Autopick to have a solid basis and adjust accordingly. Playmakers work better in midfield and throw-ins should definitely not take out a forward. Have corners taken by cards with weak heading stats.
An important thing not to miss is putting in substitutes of the same league that are still capable, if possible. Do, however, keep in mind these need to be able to participate in matches, so have as many fitting positions in there, because a card in the wrong position won't be eligible. Avoid those at all costs.
Always try to have a level 1 keeper as a sub, because they'll never be necessary and if they are, it's in one match, before a better sub can take the field. Reserve limits are always much lower and those levels can really come in handy.
Now that there's a starting formation, it's important not to dive into games blindly, right away. In the Squad tab, go to Tactics and adjust the sliders according to the team's strengths. Always pick a more attacking style, since defenses never win games. Also, United Eleven favors long passing and tough tackles. It's not ironclad, but it does seem to create a lot of opportunities
You can set automated substitution to take out fatigued positions, but you're better off just checking every few hours and taking out troubled players. Don't ever use the Manage tab to release players. They have more value in training.
With the foundation now pretty much done, feel free to take a spin in the Challenge tab and use a Daily Match or Tour game to see how good the squad is performing. Adjust Tactics from there, maybe swap players if really needed, but those should be good as long as they're the strongest Overall score possible.